Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films

ENCYCLOPEDIA OF MATERIALS CHARACTERIZATION MATERIALS CHARACTERIZATION SERIES Surfaces, Interfaces, Thin Films Serie...

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ENCYCLOPEDIA OF MATERIALS CHARACTERIZATION

MATERIALS CHARACTERIZATION SERIES

Surfaces, Interfaces, Thin Films

Series Editors: C. Richard Brundle and Charles A. Evans, Jr. Series Titles Encyclopedia of Materiah Characterization, C. Richard Brundle, Charles k Evans, Jr., and Shaun Wilson Characterizationof Mekth and Alloys, Paul.H. Holloway and P. N.

Vaidyanathan Characterizationof Ceramics, Ronald E. Loehman Characterimtion of Pobmers, Ned J. Chou, Stephen P. Kowalczyk, Ravi Sard, and Ho-Ming Tong Characterizationin Silicon Processing, Yale Strausser Characterizationin Compound Semiconductor Processing,

Yale Strausser Characterizationof Integraed CircuitPackaging Materiah,

Thomas M. Moore and Robert G. McKenna Characterizationof Cadytic Materiah, Israel E. Wachs Characterizationof Composite Materiah, Hatsuo Ishida Characterizationof OpticalMateriah, Gregory J. Exarhos Characterizationof Tribological Materiah, William A. Glaeser Characterizationof Organic Thin Films,Abraham Ulman

ENCYCLOPEDIA OF MLATERIALS CHARACTERIZATION Surfaces, Interfaces, Thin Films

EDITORS

C Ricbard Brundle CharlesA. Evans,Jr. Sbaun Wihon MANAGING EDITOR

Lee E. Fitzpatrick

BUTTERWORTH-HEINEMANN Boston London Oxford Singapore Sydney Toronto Wellington MANNING Greenwich

Thisbook was acquired, developed, and produced by Manning PublicationsCo. Copyright Q 1992 by Butxetworch-Heinemann,a division ofReed Publishing CUSA) Inc

Au rights r a d No parc ofthis publicarionmay be reproduced, scored in a retried system, or transmitted, in any form or by means. electronic, mechanical, photocopying, or orherwise, without prior written permission of the publisher. Recognizing the importanceof preserving what has been written, it is the policy of Butterworth-Heinemannand of Manning to have the books they publish printed on acid-free paper, and we exert our best &m to that end.

Library of Congress Cataloging-in-Publication Data Brundle, C. R. Encyclopediaof materials characterization: surfaces, interfaces, thin films/C. Richard Brundle, Charles A. Evans, Jr., Sham Wilson. p. un.--(Materials characterization series) Indudes bibliographical refrrenoaand index. ISBN CL7506-9168-9 1. Surfaces (Tedmoology)-Tes~ I. Evans, Charlak 11. Wilson, Shaun. 111. Title. IV. Series. TA418.7.B73 I992 92-14999 620’.4Pdc20 CIP

Butterworth-Heinemann 80 Montvale Avenue Stoneham, MA02180 Manning Publications Co. 3 h i s Street Greenwich, CT 06830 1 0 9

8

7 6 5 4 3

Printed in the Unired StatesofAmerica

Contents

Preface to Series ix Preface x Acronyms Glossary xi Contributors xvi

INTRODUCTION AND SUMMARIES 1.0

Introduction I Technique Summaries

7-56

IMAGING TECHNIQUES (MICROSCOPY) 2.0 2.1 2.2 2.3 2.4

Introduction 57 Light Microscopy 60 Scanning Electron Microscopy, SEM 70 Scanning Tunneling Microscopy and Scanning Force Microscopy, STM and SFM 85 Transmission Electron Microscopy, TEM 99

ELECTRON BEAM INSTRUMENTS 3.0 3.1 3.2 3.3 3.4 3.5

Introduction 117 Energy-DispersiveX-Ray Spectroscopy, EDS 120 Electron Energy-Loss Spectroscopy in the Transmission Electron Microscope, EELS 135 Cathodoluminescence, CL 149 Scanning Transmission Electron Microscopy, STEM Electron Probe X-Ray Microanalysis, EPMA 175

161

V

STRUCTURE DETERMINATION BY DIFFRACTION AND SCATTERING 4.0 4.1 4.2 4.3 4.4 4.5 4.6

Introduction 193 X-Ray Diffraction, XRD 198 Extended X-Ray Absorption Fine Structure, EXAFS 214 Su&ce Extended X-Ray Absorption Fine Structure and Near Edge X-Ray Absorption Fine Structure, SEXAFS/NEXAFS X-Ray Photoelectron and Auger Electron Difiction, XPD and AED 240 Low-Energy Electron Diffraction, LEED 252 Reflection High-Energy Electron Diffraction, W E E D 264

227

ELECTRON EMISSION SPECTROSCOPIES 5.0 5.1 5.2 5.3 5.4

Introduction 279 X-Ray Photoelectron Spectroscopy, X P S 282 Ultraviolet Photoelectron Spectroscopy, UPS 300 Auger Electron Spectroscopy, AES 310 Reflected Electron Energy-loss Spectroscopy, REELS 324

X-RAY EMISION TECHNIQUES 6.0 6.1 6.2 6.3

Introduction 335 X-Ray Fluorescence, XRF 338 Total Reflection X-Ray Fluorescence Analysis, TXRF 349 Particle-Induced X-Ray Emission, PIXE 357

VISIBLE/W EMISSION, REFLECTION, AND ABSORPTION 7.0 7.1 7.2 7.3

Introduction 371 Photoluminescence, PL 373 Modulation Spectroscopy 385 Variable Angle SpectroscopicEllipsometry, VASE 401

VIBRATIONAL SPECTROSCOPIES AND NMR 8.0 8.1 8.2 8.3 8.4

Introduction 413 Fourier Transform Infrared Spectroscopy, FTIR 416 RamanSpectroscopy 428 High-Resolution Electron Energy Loss Spectroscopy, HREELS 4-42 Solid State Nuclear Magnetic Resonance, NMR 460

vi Contents

ION SCATTERING TECHNIQUFS 9.0 9.1 9.2 9.3 9.4

Introduction 473 Rutherford Backscattering Spectrometry, RBS 476 Elastic Recoil Spectrometry, ERS 488 Medium-Energy Ion Scattering with Channeling and Blocking, MEIS 502 Ion scattering Spectroscopy, Iss 514

MASS AND OPTICAL SPECTROSCOPIES 10.0 Introduction 527 10.1 Dynamic Secondary Ion Mass Spectrometry, Dynamic SIMS 532 10.2 Static Secondary Ion Mass Spectrometry, Static SIMS 549 10.3 Surfice Analysis by h e r Ionization, SAL1 559 10.4 Sputtered Neutral Mass Spectrometry, SNMS 571 10.5 Laser Ionization Mass Spectrometry, LIMS 586 10.6 Spark Source Mass Spectrometry, SSMS 598 10.7 Glow-Discharge Mass Spectrometry, GDMS 609 10.8 Inductively Coupled Plasma Mass Spectrometry, ICPMS 624 10.9 Inductively Coupled Plasma-Optical

Emission Spectroscopy, ICP-OES 633

NEUTRONANDNUCLEARTECHNIQUES 1'1.0 Introduction 645 11.I Neutron Diffraction

648 11.2 Neutron Reflectivity 660 11.3 Neutron Activation Analysis, NAA 671 11.4 Nuclear Reaction Analysis, NRA 680

PHYSICAL AND MAGNETIC PROPERTIES 12.0 Introduction

695

12.1 Surface Roughness: Measurement, Formation by

Sputtering, Impact on Depth Profiling 698 12.2 Optical Scatterometry 711 12.3 Magneto-optic Kerr Rotation, MOJSE 723 12.4 Physical and Chemical Adsorption Measurement of Solid Surface Areas 736

Contents vii

Preface to Series This Materialj Characterization Series attempts to address the needs of the practical materials user, with an emphasis on the newer areas of surfice, interface, and thin film microcharacterization. The Series is composed of the leading volume, Enychpedia of Materialj Characterization,and a set of about 10 subsequent volumes concentrating on characterization of individual materials classes. In the Encyclopedia, 50 brief articles (each 10-18 pages in length) are presented in a standard format designed for ease of reader access, with straightforward technique descriptions and examples of their practical use. In addition to the articles, there are one-page summaries for every technique, introductory summaries to groupings of related techniques, a complete glossary of acronyms, and a tabular comparison of the major features of all 50 techniques. The 10 volumes in the Series on characterization of particular materials classes include volumes on silicon processing, metals and alloys, catalytic materials, integrated circuit packaging, etc. Characterization is approached from the materials user’s point of view. Thus, in general, the format is based on properties, processing steps, materials classification, etc., rather than on a technique. The emphasis of all volumes is on surfaces, interfaces, and thin films, but the emphasis varies depending on the relative importance of these areas for the materials class concerned. Appendixes in each volume reproduce the relevant one-page summaries from the Encyclopedia and provide longer summaries for any techniques referred to that are not covered in the Envcbpedia The concept for the Series came firom discussion with Marjan Bace of Manning Publications Comparly. A gap exists between the way materials characterization is often presented and the needs of a large segment of the audience-the materials user, process engineer, manager, or student. In our experience, when, at the end of talks or courses on analytical techniques, a question is asked on how a particular material (or processing) characterization problem can be addressed the answer often is that the speaker is “an expert on the technique, not the materials aspects, and does not have experience with that particular situation.” This Series is an attempt to bridge this gap by approaching characterization problems from the side of the materials user rather than from that of the analytical techniques expert. We would like to thank Marjan Bace for putting forward the original concept, Shaun Wilson of Charles Evans and Associates and Yale Strausser of Surface Science Laboratories fbr help in hrther defining the Series, and the Editors of all the individual volumes for their efforts to produce practical, materials user based volumes.

C R Brundle C.A. Evans,Jr.

ix

This volume contains 50 articles describing analytical techniques for the characterization of solid materials, with emphasis on surfaces, intedices, thin films, and microanalytical approaches. It is part of the Materzah CharacterizationSeries, copublished by Buttenvorth-Heinemann and Manning. This volume can serve as a stand-alone reference as well as a companion to the other volumes in the Series which deal with individual materials classes. Though authored by professional characterization experts the articles are written to be easily accessible to the materials user, the process engineer, the manager, the student-in short to all those who are not (and probably don’t intend to be) experts but who need to understand the potential applications of the techniques to materials problems. Too often, technique descriptions are written for the technique specialist. With 50 articles, organization of the book was difficult; certain techniques could equally well have appeared in more than one place. The organizational intent of the Editors was to group techniques that have a similar physical basis, or that provide similar types of information. This is not the traditional organization of an encyclopedia, where articles are ordered alphabetically. Such ordering seemed less useful here, in part because many of the techniques have multiple possible acronyms (an Acronym Glossavy is provided to help the reader). The articles follow a standard format for each technique: A clear description of the technique, the range of information it provides, the range of materials to which it is applicable, a few typical examples, and some comparison to other related techniques. Each technique has a “quick reference,” one-page summary in Chapter 1,consisting of a descriptive paragraph and a tabular summary. Some of the techniques included apply more broadly than just to surhces, interhces, or thin films; for example X-Ray Diffraction and Infrared Spectroscopy, which have been used for half a century in bulk solid and liquid analysis, respectively. They are included here because they have by now been developed to also apply to surfaces. A fay techniques that are applied almost entirely to bulk materials (e.g., Neutron Diffraction) are included because they give complementary information to other methods or because they are referred to significantly in the 10 materials volumes in the Series. Some techniques were left out because they were considered to be too restricted to specific applicationsor materials. We wish to thank all the many contributors for their efforts, and their patience and restraint in dealing with the Editors who took a hirly demanding approach to establishing the format, length, style, and content of the articles. We hope the readers will consider our efforts worthwhile. Finally, we would like to thank Lee Fitzpatrick of Manning Publications Co. for her professional help as Managing Editor.

C.R.Brund. CA. Evans,/r. S. Mhon

Acronyms Glossary This glossary lists all the acronyms referred to in the encyclopedia together with their meanings. The major technique acronyms are listed alphabetically. Alternatives to these acronyms are listed immediately below each of these entries, if they exist. Related acronyms (variations or subsets of techniques; terminology used within the technique area) are grouped together below the major acronym and indented to the right. Most, but not all, of the techniques listed here are the subject of individual articles in this volume.

AAS AA VPD-AAS GFAA FAA

AES Auger

SAM SAM AED

ADAM K.E CMA

Atomic Absorption Spectroscopy Atomic Absorption Vapor Phase Decomposition-AtomicAbsorption Spectroscopy Graphite Furnace Atomic Absorption Flame Atomic Absorption Auger Electron Spectroscopy Auger Electron Spectroscopy Scanning Auger Microscopy Scanning Auger Microprobe Auger Electron Diffraction Angular Distribution Auger Microscopy Kinetic Energy Cylindrical Mirror Analyzer

AIS

Atom Inelastic Scattering

BET

Brunauer, Emmett, and Teller equation

BSDF BRDF BTDF

Bidirectional Scattering Distribution Function BidirectionalReflective Distribution Function BidirectionalTransmission Distribution Function

CL

Cathodluminescence

CLSM

Confocal Scanning Laser Microscope

EDS EDX EDAX

Energy Dispersive (X-Ray) Spectroscopy Energy Dispersive X-Ray Spectroscopy Company selling EDX equipment

EELS HEELS REELS REELM EELS

Electron Energy Loss Spectroscopy High-Resolution Electron Energy-Loss Spectroscopy Reflected Electron Energy-Loss Spectroscopy Reflection Electron Energy-Loss Microscopy Low-Energy Electron-Loss Spectroscopy xi

PEELS EXELFS EELFS CEELS VEELS

Parallel (Detection) Electron Energy-Loss Spectrscopy Extended Energy-Loss Fine Structure Electron Energy-Loss Fine Structure Core Electron Energy-Loss Spectroscopy Valence Electron Energy-Loss Spectroscopy

EPMA Electron Probe

Electron Probe Microanalysis Electron Probe Microanalysis

ERS HFS HRS FRS ERDA ERD PRD

Elastic Recoil Spectrometry Hydrogen Forward Scattering Hydrogen Recoil Spectrometry Forward Recoil Spectrometry Elastic Recoil Detection Analysis Elastic Recoil Detection Particle Recoil Detection

EXAFS

Extended X-Ray Absorption Fine Structure Surface Extended X-Ray Absorption Fine Structure Near-Edge X-Ray Absorption Fine Structure X-Ray Absorption Near-Edge Structure X-Ray Absorption Fine Structure

SEXAFS NEXAFS XANES

XAFS FMR

Ferromagnetic Resonance

FTIR

See IR

FT Raman

See Raman

HREELS

See EELS

HRTEM

See TEM

GDMS GDQMS

Glow Discharge Mass Spectrometry Glow Discharge Mass Spectrometryusing a Quadruple Mass Analyser Manufacturer name

Gloquad ICP-MS ICP LA-ICP-MS

Inductively Coupled Plasma Mass Spectrometry Inductively Coupled Plasma Laser Ablation ICP-MS

ICP-Optical ICP

InductivelyCoupled Plasma Optical Emission InductivelyCoupled Plasma

IETS

Inelastic Electron Tunneling Spectroscopy

IR

Infrared (Spectroscopy) Fourier Transform Infra-Red (Spectroscopy) Gas Chromatography FTIR Thermo GravimetricAnalysis FTIR ArtenuatedTotal Reflection

FTIR GC-FTIR TGA-FTIR ATR xii Acronyms Glossary

RA IRAS

Reflection Absorption (Spectroscopy) Infrared Reflection Absorption Spectroscopy

ISS LEIS RCE

Ion Scattering Spectrometry Low-Energy Ion Scattering Resonance Charge Exchange

LEED

Low-Energy Electron Diffraction

LIMS LAMMA LAMMS LIMA NRMPI

Laser Ionization Mass Spectrometry Laser Microprobe Mass Analysis Laser Microprobe Mass Spectrometry Laser Ionization Mass Analysis Nonresonant Multi-Photon Ionization

MEISS MEIS

Medium-Energy Ion Scattering Spectrometry Medium-Energy Ion Scattering

MOKE SMOKE

Magneto-optic Kerr Rotation Surface Magneto-optic Kerr Rotation

NAA INAA

Neutron Activation Analysis Instrumental Neutron Activation Analysis

NEXAFS XANES

Near Edge X-Ray Absorption Fine Structure X-Ray Absorption Near Edge Structure

NIS

Neutron Inelastic Scattering

NMR MAS

Nuclear Magnetic Resonance Magic-Angle Spinning

NRA

Nuclear Reaction Analysis

OES

Optical Emission Spectroscopy

PAS

Photoacoustic Spectroscopy

PIXE

Particle Induced X-Ray Emission Hydrogen/Helium Induced X-ray Emission

HIXE PLE

Photoluminescence PhotoluminescenceExcitation

EBER RDS

Photoreflectance Electron Beam Electroreflectance Reflection Difference Spectroscopy

PL PR

Raman FT Raman RS RRS CARS

Raman Spectroscopy Fourier Transform Raman Spectroscopy Raman Scattering Resonant Raman Scattering Coherent Anti-Stokes Raman Scattering

Acronyms Glossary xiii

Surface Enhanced Raman Spectroscopy

SERS HEIS

Rutherford BackscatteringSpectrometry High-Energy Ion Scattering

WEED SREM

Reflected High Energy Electron Diffraction Scanning Reflection Electron Microscopy

SAL1 PISIMS MPNRPI MRRPI RPI MPI SPI SINS SARIS TOFMS

SurficeAnalysis by h e r Ionization

Post-Ionization Secondary Ion Mass Spectrometry Multi-Photon Nonresonant Post Ionization Multiphoton Resonant Post Ionization Resonant Post Ionization Multi-Photon Ionization Single-Photon Ionization Sputter-Initiated Resonance Ionization Spectroscopy Surface Analysii by Resonant Ionization Spectroscopy Time-of-Flight Mass Spectrometer

SAM

See AES

SEM

ScanningElectron Microscopy ScanningElectron Microprobe Secondary Electron Miscroscopy SecondaryElectron Backscattered Electron Secondary Electron Microscopywith PolarizationAnalysis Scanning Force Microscopy Scanning Force Microscope

RBS

SE BSE SEMPA SFM AFM SPM

Atomic Force Microscopy Scanning Probe Microscopy

SIMS Dynamic SIMS Static SIMS Q-SIMS Magnetic SIMS Sector SIMS TOF-SIMS PISIMS

Secondary Ion Mass Spectrometry Dynamic Secondary Ion Mass Spectrometry Static Secondary Ion Mass Spectrometry SIMS using a Quadruple Mass Spectrometer SIMS using a Magnetic Sector Mass Spectrometer See Magnetic SIMS SIMS using Tune-of-Flight Mass Spectrometer Post Ionization SIMS

SNMS

Sputtered Neutrals Mass Spectrometry Secondary Neutrals M a s s Spectrometry Direct Bombardment Electron Gas SNMS

SNMSd SSMS Spark Source

Spark Source Mass Spectrometry Spark Source Mass Spectrometry

STEM

See TEM

STM

ScanningTunneling Microscopy

xiv Acronyms Glossary

SPM

ScanningTunneling Microscope Scanning Probe Microscopy

TEAS

Thermal Energy Atom Scattering

TEM

Transmission Electron Microscopy Transmission Electron Microscope Conventional Transmission Electron Microscopy ScanningTransmission Electron Microscopy High Resolution Transmission Electron Microscopy Selected Area Diffraction Analytical Electron Microscopy Convergent Beam Electron Diffraction Lorentz Transmission Electron Microscopy

CTEM STEM HRTEM SAD AEM CBED LTEM TLC

Thin Layer Chromatography

TSRLM TSM

Tandem Scanning Reflected-Light Microscope Tandem Scanning Reflected-Light Microscope

TXRF

See XRF

UPS MPS

Ultraviolet Photoelectron Spectroscopy Ultraviolet Photoemission Spectroscopy Molecular Photoelectron Spectroscopy

VASE

VariableAngle SpectroscopicEllipsometry

WDS WDX

Wavelength Dispersive &-Ray) Spectroscopy Wavelength Dispersive X-Ray Spectroscopy

XAS

X-Ray Absorption Spectroscopy

XPS

X-Ray Photoelectron Spectroscopy X-Ray Photoemission Spectroscopy Electron Spectroscopyfor Chemical Analysis X-Ray Photoelectron Diffraction Photoelectron Diffraction Kinetic Energy

ESCA XPD PHD KE

XRD GIXD GIXRD DCD

XRF XFS TXRF TRXFR VPD-TXRF

X-RayDiffraction Grazing Incidence X-Ray Diffraction Grazing Incidence X-Ray Diffraction Double Crystal Diffractometer X-Ray Fluorescence X-Ray Fluorescence Spectroscopy Total Reflection X-Ray Fluorescence Total Reflection X-Ray Fluorescence Vapor Phase Decomposition Total X-Ray Fluorescence

Acronyms Glossan/ xv

Mark R Antonio

Extended X-Ray Absorption Fine Structure

BP Research International Cleveland, OH J. E. E. Bagh IBM Alrnaden Research Center San Jose, CA

Elastic Recoil Spectrometry

Scott Baumann Charles Evans &Associates Redwood City, CA

Rutherford BackscatteringSpectrometry

Christopher H. Becker SRI International Menlo Park, CA

SurfaceAnalysis by Laser Ionization

Albert J. Bevolo Ames Laboratory, Iowa State University Ames, IA

Reflected Electron Energy-Loss Spectro~copy

J. B. Bindell AT&T Bell Laboratories Allentown, PA

Scanning Electron Microscopy

Filippo Radicati di Brozolo Charles Evans &Associates Redwood City, CA

Laser Ionizarion Mass Spectrometry

C. R Brundle

X-Ray Photoelectron Spectroscopy; Ultraviolet Photoelectron Spectroscopy

IBM Almaden Research Center San Jose, CA Daniele Cherniak Rennsselaer PolytechnicInstitute Troy, NY

Nuclear Reaction Analysis

Paul Chu Charles Evans & Associates Redwood City, CA

Dynamic Secondary Ion Mass Spectrometry

Carl Colvard

Photoluminescence

Charles Evans & Assoociates Redwood City, CA J. Neal Cox INTEL, Components Research Santa Clara, CA

Fourier Transform Infrared Spectroscopy

John Gustav Delly McCrone Research Institute Chicago, IL

Light Microscopy

xvi

Hellmut F.ckert University of California, Santa Barbara Santa Barbara, CA

Solid State Nuclear Magnetic Resonance

Peter Eichinger GeMeTecAnalysis Munich

Total Reflection X-Ray Fluorescence

P. Fenter Rutgers University Piscataway, NJ

Medium-Energy Ion Scatteringwith Channeling and Blocking

David E. Fowler IBM Almaden Research Center San Jose, CA

Magneto-optic Kerr Rotation

S. M. Gaspar University of New Mexico Albuquerque,NM

Optical Scatterometry

ROYH. Geiss IBM Almaden Research Center San Jose, CA

Energy-Dispersive X-Ray Spectroscopy

Torgny Gustafsson Rutgers University Piscataway, NJ

Medium-Energy Ion ScattefmgWith Channeling and Blocking

William L. Harrington Evans Fast Plainsboro, NJ

Spark Source Mass Spectromeuy

Brent D. Hermsmeier IBM Almaden Research Center San Jose, CA

X-Ray Photoelectronand Auger Electron Diffraction

K.C. Hidunan University of New Mexico Albuquerque,NM


Tim Z. Hossain Cornell University Ithica, NY

Neutron Activation Analysis

ScanningTunneling Microscopy

Rebecca S. Howland Park Scientific Instruments Sunnyvale, CA

and ScanningForce Microscorn

John C. Huneke Charles Evans & Assodates

SputteredNeutral Mass Spectrometry, Glow-DischargeMass Spectrometry

Redwood City, CA Ting C. Huang IBM Almaden Research Center San Jose, CA

X-Ray Fluorescence

William Katz Evans Central Minnetonka,MN

Static Secondary Ion Mass Spectrometry

Michael D. Kirk Park Scientific Instruments Sunnyvale, CA

ScanningTunneling Microscopy and Scanning Force Microscopy

Contributors xvii

Bruce E. Koel University of Southern California Los Angles, CA

High-Resolution Electron Energy Loss Spectrometry

Max G. Lagally University of Wmnsin, Madison, WI

Low-Energy Electron Diffraction

W. A. Lanford State University ofNew York, Albany, NY

Nudear Reaction Analysis

Charles E. Lyman Lehigh University Bethlehem, PA

ScanningTransmission Electron Microscopy

Susan MacKay

Surface Analysis by Laser Ionization

Perkin Elmer Eden Prairie, MN John R McNeil University of New Mexico Albuquerque,NM


Ronald G. Musket Lawrence Livermore National Laboratory Livermore, (=A

Partide-InducedX-Ray Emission

S. S. H. Naqvi Universiy of New Mexico Albuquerque,NM


Dale E. Newbury National Institutes of Science and Technology Gairhersburg, MD

Electron Probe X-Ray Microanalysis

David Norman SERC Daresbury Laboratory Daresbury, Cheshire

Surface Extended X-Ray Absorption Fine Structure, Near Edge X-Ray Absorption Fine Structure

John W. Olesik Ohio State University Columbus, OH

InductivelyCoupled Plasma-Optical Emission Spectroscopy

Fred H. Pollak Brooklyn College, CUNY New York, NY

Modulation spectroscopy

Thomas P. Russell IBM A l d e n Research Center San Jose, CA

Neutron Reflectivity

Donald E. Savage University of Wisconsin Madison, WI

Reflection High-EnergyElectron Diffraction

Kurt E. Si& Los Alamos National Laboratory LosAlamos,NM

Transmission E l m n Microscopy

Paul G. Snyder University of Nebraska Lincoln, NE

Variable Angle Spectroscopic Ellipsometry

xviii Contributors

Gene Sparrow Advanced R&D St. Paul, MN

Ion Scattering Spectroscopy

Fred A. Stevie AT&T Bell Laboratories Allentown, PA

Surface Roughness: Measurement, Formation by Sputtering, Impact on Depth Profiling

Yale E. Strausser Surface Science Laboratories Mountainview, CA

Auger Electron Spectroscopy

Barry J. Streusand Applied Analytical Austin, TX

Inductively Coupled Plasma Mass Spectrometry

Raymond G. Teller BP Research International Cleveland, OH

Neutron Diffraction

Michael F. Toney IBM Almaden Research Center San Jose, CA

X-Ray Diffraction

Wojciech Viech Charles Evans & Assoociates Redwood City, CA

Glow-Discharge Mass Spectrometry

William B. White Pennsylvania Skate UniVeI'Siq' University Park, PA

Raman Spectroscopy

S.R Wilson University of New Mexico Albuquerque, NM


John A. Woollam University of Nebraska Lincoln, NE

Variable Angle SpectroscopicEllipsometry

Ben G. Yacobi University of California at Los Angeles Los Angeles, CA

Cathodoluminescence

David J. C. Yates Consultant Poway, CA

Physical and Chemical Adsorption for the Measurement of Solid SurfaceAreas

Nestor J. Zaluzec Argonne National Laboratory Argonne, IL

Electron Energy-Loss Spectroscopy in the Transmission Electron Microscope

Contributors xix

I INTRODUCTION AND SUMMARIES

1.0

INTRODUCTION

Though a wide range of analytical techniques is covered in this volume there are certain traits common to many of them. Most involve either electrons, photons, or ions as a probe beam striking the material to be analyzed. The beam interacts with the material in some way, and in some of the techniques the changesinduced in the beam (energy, intensity, and angular distribution) are monitored after the interaction, and analytical information is derived from the observation of these changes. In other techniques the information used for analysis comes from electrons, photons, or ions that are ejected from the sample under the stimulation of the probe beam. In many situations several connected processes may be going on more or less simultaneously, with a particular analytical technique picking out only one aspect, e.g., the extent of absorption of incident light, or the kinetic energy distribution of ejected electrons. The range of information provided by the techniques discussed here is as0 wide, but again there are common themes. What types of information are provided by these techniques? Elemental composition is perhaps the most basic information, followed by chemical state information, phase identification, and the determination of structure (atomic sites, bond lengths, and angles). One might need to know how these vary as a function of depth into the material, or spatiallyacross the material, and many techniques specialize in addressingthese questions down to very fine interfaces, and thin films there is ofien very little material dimensions. For su&s, at all to analyze, hence the presence of many microanalytical methods in this volume. Within this field (microanalysis) it is o b necessary to identify trace components down to extremely low concentration (parts per trillion in some cases) and a number of techniques specialize in this aspect. In other cases a high degree of accuracy in measuring the presence of major components might be the issue. Usually the techniques that are good fbr trace identification are not the same ones used to accurately quantify major components. Most complete analyses require the use of 1

multiple techniques, the selection of which depends on the nature of the sample and the desired information. This first chapter contains onepage summaries of each of the 50 techniques covered in the following chapters. All summaries have the same format to allow easy comparison and quick access to the information. Further comparative information is provided in the introductions to the chapters. Finally, a table is provided at the end ofthis introduction, in which many ofthe important parametersdescribing the capabilities for all 50 techniques are listed. The subtitle of this Series is “Su&m, Interfices, and Thin Films.” The definition of a “surface” or of a “thin film” varies considerably h m person to person and with application area. The academic discipline of ‘‘Surfice Science” is largely concerned with chemistry and physics at the atomic monolayer level, whereas the “surface region” in an engineering or applications sense can be much more extensive. The same is true for interfaces between materials. The practical consideration in distinguishing “sudace” from “bulk” or “thin” from “thick” is usually connected to the property of interest in the application. Thus, fbr a catalytic reaction the presence of haf a monolayer of extraneous sulfur atoms in the top atomic layer of the catalyst material might be critical, whereas for a corrosion protection layer (for example, Cr segregation to the surface region in steels) the important region of depth may be several hundred 8,. For interfaces the epitaxial relationship between he last atomic layer of a single crystal material and the first layer of the adjoining material may be critical for the electrical properties of a device, whereas diffusion barrier interfaces elsewhere in the same device may be 1000 A thick. In thin-film technology requirements can range from layers pm thick, which for the majority of analytical techniques discussed in this volume constitute bulk material, to layers as thin as 50 8, or so in thin-film magnetic recording technology. Because of these different perceptions of “thick” and “thin,” actual numbers are used whenever discussing the depth an analytical technique examines. Thus in Ion Scattering Spectroscopy the signals used in the analysis are generated fiom only the top atomic monolayer of material exposed to a vacuum, whereas in X-ray photoemission up to 100 8,is probed, and in X-ray fluorescence the signal can come from integrated depths ranging up to 10 pm. Note that in these three examples, two are quoted as having ranges of depths. For many of the techniques it is impossible to assign unique values because the depth from which a signal originates may depend both on the particular manner in which the technique is used, and on the nature of the material being examined. Perfbrming measurements at grazing angles of incidence of the probe beain, or grazing exit angles fbr the detected signal, will usually make the technique more surface sensitive. For techniques where X-ray, electron, or high-energy ion scattering is the critical Factor in determining the depth analyzed, materials consisting of light elements are always probed more deeply than materials consisting of heavy elements. 2

INTRODUCTION AND SUMMARIES

Chapter 1

Another confusing issue is that of “depth resolution.” It is a measurement of the technique’s ability to clearly distinguish a property as a function of depth. For example a depth resolution of 20 A, quoted in an elemental composition analysis, means that the composition at one depth can be distinguished from that at another depth if there is at least 20 between them. A depth profile is a record of the variation of a property (such as composition) as a function of depth. Some of the techniques in this volume have essentially no intrinsic depth profiling capabilities; the signal is representative of the material integrated over a fived probing depth. Most, however, can vary the depth probed by varying the condition of analysis, or by removing the surface, layer by layer, while collecting data. By varying the angle of incidence, the X-ray, electron, or ion beam energy, etc. many techniques are capable of acquiring depth profiles. Those profiles are generated by combining several measurements, each representative of a different integrated depth. The higher energy ion scattering techniques (Medium Energy Ion Scattering, MEIS, and Rutherford Backscattering, RBS), however, are unique in that the natural output of the methods is composition as a function of depth. By far the most common way of depth profiling is the destructive method of removing the surface, layer by layer, while also taking data. For the mass spectrometry-based techniques of Chapter 10, removal of surface material is intrinsic to the sputtering and ionization process. Other methods, such as Auger Electron Spectroscopy,AES, or X-Ray Photoemission, X P S , use an ancillary ion beam to remove material while constantly ionizing the newly exposed surface. Under the most favorable conditions depth resolutions of around 20 A can be achieved this way, but there are many artifacts to be aware of and the depth resolution usually degrades rapidly with depth. Some aspects of sputter depth profiling are touched upon in the article “Surface Roughness” in Chapter 12, but for a more complete discussion of the capabilities and limitations of sputter depth profiling the reader is referred to a paper by D. Marton and J. Fine in Thin Solid Films, 185, 79, 1990 and to other articles cited there.

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Compilationof Comparative Informationon the Analytical Techniques Discussed in This Volume

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2.1 2.2 2.3 2.3 2.4 3.1 3.2 3.3 3.4 3.5 4.1 4.2 4.3 4.3 4.4 4.5 4.6 5.1 5.2 5.3

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Compilationof Comparative Information on the AnalyticalTechniques Discussedin This Volume

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All: z as well as the use of CCD array-TV displays to enhance detection sensitivity. TEM Operation

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern: a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. The examples in Figure 3 illustrate these possibilities. Figure 3a shows a diffraction pattern from a single crystal Fe thin film, oriented with the [OOl] crystal axis

104

IMAGING TECHNIQUES

Chapter 2

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(a) Diffractionpatternfrom a single crystal Fe thin film, oriented with the [OOl] crystal axis parallel to the incident electron beam direction. (b) Diffraction pattern from a polycrystalline thin film of Pdsi. (c) Diffraction pattern from the same film as in (c), following irradiation of the film with 400-keV Kr* ions. See text for discussion (b, c Courtesy of M. Nastasi, Los Alamos National Laboratory)

parallel to the incident electron beam direction. This single crystal produces a characteristic spot pattern. In this case, the four-fold symmetry of the difhction pattern is indicative of the symmetry of this body-centered cubic lattice. Figure 3b shows a ring pattern from a polycrystalline thin film, Pd2Si. Figure 3c shows a diffuse halo diffraction pattern from the same film, following irradiation of the film with 400-keV Kr+ ions. The d i h e halos (the second-order halo here is very fiint) are indicative of scattering from an amorphous material, demonstrating a dramatic disordering of Pd2Si crystal lattice by the Kr+ ions. The image mode produces an image of the illuminated sample area, as in Figure 2. The image can contain contrast brought about by several mechanisms: mass contrast, due to spatial separations between distinct atomic constituents; thickness contrast, due to nonuniformity in sample thickness; diffraction contrast, which in the case of crystallinematerials results from scattering of the incident electron wave by structural defects; and phase contrast (see discussion later in this article). Alternating between image and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 2.4

TEM

105

Electron Optics

It is easiest to discuss the electron optics of a TEM instrument by addressing the instrument from top to bottom. Refer again to the schematic in Figure la. At the top of the TEM column is an electron source or gun. An electrostaticlens is used to accelerate electrons emitted by the filament to a high potential (typically 1001,000 kV) and to focus the electrons to a cross-over just above the anode (the diameter of the cross-over image can be from 0.5 to 30 p,depending on the type of gun4). The electrons at the cross-over image of the filament are delivered to the specimen by the next set of lenses on the column, the condensers. Most modern TEMs use a two-stage condenser lens system that makes it possible to i Produce a highly demagnified image of cross-over at the specimen, such that only a very small sample region is illuminated (typically e 1 pm) . 2 Focus the beam at “infinity” to produce nearly parallel illumination at the specimen. The former procedure is the method of choice during operation in the image mode, while the latter condition is desirable for maximizing source coherency in the diffraction mode. The specimen is immersed in the next lens encountered along the column, the objective lens. The objective lens is a magnetic lens, the design of which is the most crucial of all lenses on the instrument. Instrumental resolution is limited primarily by the spherical aberration of the objective lens. The magnetic field at the center of the objective lens near the specimen position is large, typically 2-2.5 T (20-25 kG).*This places certain restrictions on TEMs applicability to studies of magnetic materials, particularly where high spatial resolution measurements are desired. Nevertheless, low-magnification TEM is often used to study magnetic domain characteristics in magnetic materials, using so-called hrentz microscopy procedures.5 In such instances, the objective lens is weakly excited, so that the incident electrons “see” mainly the magnetic field due to the specimen. Changes in this field across domain boundaries produce contrast in the transmitted image. The final set of magnetic lenses beneath the specimen are jointly referred to as post-specimen lenses. Their primary task is to magnify the signal transferred by the objective lens. Modern instruments typically contain four post-specimen lenses: diffraction, intermediate, projector 1, and projector 2 (in, order of appearance below the specimen). They provide a TEM with its tremendous magnification flexibility. Collectively, the post-specimen lenses serve one of two purposes: they magnify either the diffraction pattern from the sample produced at the back focal plane of the objective lens; or they magnrfi. the image produced at the image plane of the objective lens. These optical planes are illustrated in the electron ray diagram in 106

IMAGING TECHNIQUES

Chapter 2

Figure lb. By varying the lenses’ strengths so as to alternate between these two object planes, the post-specimen lenses deliver either a magnified diffraction pattern or a magnified image of the specimen to the detector. The primary remaining considerations regarding the TEM column are the diaphragms or apertures employed at certain positions along the column. The purpose of these apertures is to filter either the source or the transmitted signal. The most important diaphragm is called the objective aperture. This aperture lies in the back focal plane of the objective lens. In this plane the scattered electron waves recombine to form a diffraction pattern. A diffraction pattern corresponds to the angular dispersion of the electron intensity removed from the incident beam by interaction with the specimen. Inserting an aperture in this plane effectivelyblocks certain scattered waves. The larger the objective aperture, the greater the angular dispersion that is accepted in the transmitted signal. Figure 1b shows an example where the undeflected or transmitted beam is passed by the objective aperture, while the firstorder, Bragg-diffracted beam is blocked. Consequently, only intensity in the transmitted beam can contribute to the image formed at the image plane of the objective lens. Use of a small objective aperture while operating in the image mode, which blocks all diffracted beams (as in this example), can serve to enhance significantly image contrast. Use of a large objective aperture, that allows the passage of many diffracted beams, is the modus operandifor the technique referred to as high-resolution transmission electron microscopy (HRTEM), discussed later in this article. Diffraction Mode

A TEM provides the means to obtain a diffraction pattern from a small specimen area. This diffraction pattern is obtained in diffraction mode, where the post-specimen lenses are set to examine the information in the transmitted signal at the back focal plane of the objective lens. Figure 4 illustrates some of the important aspects of difiaction in TEM. Figure 4a shows a micrograph obtained in image mode of a small region of a N i f l sample illuminated by an electron beam, containing lamellar crystallites with welldefined orientation relationships. Figure 4b shows a selected-area diffraction (SAD) pattern from the same region. In SAD, the condenser lens is defocused to produce parallel illumination at the specimen and a selected-area aperture (see Figure 1b) is used to limit the diffracting volume. Many spots, or reflections, are mident in this pattern, due in part to the special orientation of the sample. The S A D pattern is a superposition of diffraction patterns from crystallites in the illuminated area that possess distinct orientations. Figures 4c and 4d illustrate what happens when the incident electron probe is focused to illuminate alternately a crystallite in the cenrer of the image (labelled twin) (Figure 4c) and another crystallite adjacent to the twin (Figure 4d). This focused-probe technique is sometimes referred to as micro-dfiaction. Two effects are evident in these micro-diffraction patterns. First, the diffraction patterns consist 2.4

TEM

107

Figure 4

(a) Bright-field image from a small region of a Ni,AI sample containing oriented crystallites in the center of the illuminated area (one crystallite is labeled twin on the micrograph). (b) Selected-area diffraction (SAD) pattern from the same region as in (a). (c) Microdiffraction pattern from the middle region in (a) containing the twin crystallite. (d) Microdiffraction pattern from a crystallite adjacent t o the twin in (a, c). (Courtesy of G. T. Gray 111, Los Alamos National Laboratory)

of “discs”instead of spots. This is a consequence of the use of focused or convergent illumination instead of parallel illumination. Second, the number of reflections in each of these patterns is reduced from that of the SAD pattern in Figure 4b (the reflections are no longer paired). But a superposition of the reflected discs in the microdiffraction patterns can account for all the reflections observed in the SAD pattern. This illustrates the flexibility of a TEM to obtain diffraction information 108

IMAGING TECHNIQUES

Chapter 2

from exceedingly small areas of a sample (in this case, a region of diameter about 0.5 pn or less). The example in Figure 4 illustrates that the diffraction pattern produced by a crystalline specimen depends on the orientation of the crystal with respect to the incident beam. This is analogous to the way a Laue pattern varies upon changing the orientation of a diffracting crystal relative to an X-ray source.6 In TEM, this orientation may be varied using the sample manipulation capabilities of a tilting specimen holder. Holders come with a range of tilt capabilities, including single-axis tilt, double-axis tilt, and tilt-rotare stages, with up to S O ” tilting capabilities. But the higher the resolution of the instrument, the more limited the tilting capabilities of a tilt stage (to as low as f10”). For studies of single crystals or epitaxial thin films, it is important to have access to as much tilt capability as possible. SAD patterns often are used to determine the Bravais lattice and lattice parameters of crystalline materials. Lattice parameter measurements are made by the same procedures used in X-ray diffraction.6 Using SAD, each diffracted scattering angle 8 is measured in an S A D pattern and an associated atomic interplanar spacing d determined using Brag’s Law, h = 2d sin 8. Note that at the smdl electron wavelengths of TEM, typical 8 values are small quantiries, only 9 mrad for a Au (200) reflection using 100-keVelectrons (h = 0.0037 nm). By comparison, in a LEED experiment using 150 eV electrons, since h = 0.1 nm, a Au (200) reflection would appear at 8 = 500 mrad or 30”, using h = d sin 8; such a large scatteringangle is easily observed using the optics of a LEED system, which uses no magnifying lenses for the scattered electrons. Because of the extremely small angle scattering situation in TEM, observation of diffraction patterns is made possible only with the use of magnifying, post-specimen lenses. These lenses greatly magnify the diffraction pattern. The crystal group or Bravais lattice of an unknown crystalline material can also be obtained using SAD. This is achieved easily with polycrystalline specimens, employing the same powder pattern “indexing”procedures as are used in X-ray diffraction.6 Image Mode

In image mode, the post-specimen lenses are set to examine the information in the transmirted signal at the image plane of the objective lens. Here, the scattered electron waves finally recombine, forming an image with recognizable details related to the sample microstructure (or atomic structure). There are three primary image modes that are used in conventional TEM work, bright-field microscopy, dark-field microscopy, and high-resolution electron microscopy. In practice, the three image modes difkr in the way in which an objective diaphragm is used as a filter in the back focal plane. In bright-field microscopy, a small objective aperture is used to block all diffracted beams and to pass only the transmitted (undiffracted)electron beam. In the 2.4

TEM

109

absence of any microstructural defects, a bright-field image of a strain-free, singlephase material of uniform thickness,would lack contrast regardless of specimen orientation. Contrast arises in a bright-field image when thickness or compositional variations or structural anomalies are present in the illuminated sample area (or when the sample is strained), so that electrons in some areas are scattered out of the primary beam to a greater extent than in neighboring regions. w o n s in which intensity is most effectively removed from the incident beam to become scattered or diffracted intensity appear dark in a bright-field image since this intensity is removed by the objective diaphragm. The images in Figure 2 were obtained using the bright-field imaging procedures. In multiphase, amorphous or glassy materials, regions containing a phase of high average Z will scatter electrons more efficiently and to higher angles than regions containing a low average 2 The objective aperture in bright-field blocks this scattered intensity, making the high-Zmaterial appear darker (less transmitted intensity) than the low-Zmaterial. This is m 5 contrast, due primarily to incoherent elastic scattering. The scattering is largely incoherent because spatial relationships between scattering centers in these materials are not periodic. A priori there are no well-defined phase relationships between electrons scattered by such materials. Under these circumstances, the transmitted intensity distribution is determined from the principle of the additivity of individual scattered intensities, without consideration for the individual scattered amplitudes. In crystalline materials, dark contrast regions in bright-field usually originate from areas that are aligned for Bragg diffraction. Here, intensity is removed from the transmitted beam to produce diffracted intensity, that subsequently is blocked by the objective aperture. This is dfiaction contrast, due to coherent elastic scattering. The scattering is coherent because of the periodic arrangement of scattering centers in crystalline materials. In this case, the transmitted intensity distribution depends on the superposition of the individual scattered amplitudes. Diffraction contrast is often observed in the vicinity of defects in the lattice. The origins of this contrast are illustrated in Figure 5. Figure 5a shows a thin sample with atomic planes that are close to a Bragg diffraction orientation, but are actually unaligned with respect to an electron beam propagating down the optic axis of the microscope. O n the lefthand side of the diagram, the atomic planes are undistorted, as they would be in a perfect crystal. On the righthand side of the diagram, the sample contains an edge dislocation in the middle of the sample thickness. The dislocation lies normal to the page so that it appears in this diagram in cross section. Near the core of the dislocation, the atomic planes are distorted or bent to accommodate the strains associated with the atomic displacementsat the dislocation core. See Figure 5a. The result of these local distortions is that some planes near the core adopt a Bragg orientation with respect to the incident beam. This is shown schematicallyin Figure 5b, where the incident and transmitted electron ray paths are shown for the same sample region. The undistorted crystal on the lefthand side, which is not in 110

IMAGING TECHNIQUES

Chapter 2

\

Perfect Crystal

E e Dislocation

c$e

a Incident Electrons

Transmitted Intensity

Figure 5

i

Optic Axis

Incident Electrons

Transmitted Intensity

(a) Schematic of a thin sample with atomic planes that a n close to a b g g dfhaction orientation, but which are unaligned with respect t o an dectron beam propagating down the optic axis of the microscope. The sample contains an edge dislocation in the middle of the sample thickness on the righthand side of the diagram. (b) Incident and transmitted electron ray paths for electron scattering from the same sample region in (a). (c) Bright-field image of dislocations in shock-deformed N&AI. (d) Dark-field image from the same region as in (c). (c, d courtesy of H. W. Sizek, Los Alamos National Laboratory)

Bragg alignment, is shown as simply transmitting a similar magnitude of undeflected intensity. The region containing the dislocation, on the other hand, is 2.4

TEM

111

shown with a deficiency of undeflected, transmitted intensity, because considerable Bragg diffraction occurs near the core of the dislocation. Diffraction is represented by the scattered rays shown in the diagram, which are subsequently blocked by the objective aperture in the bright-field mode. In a bright-field image from this sample, the region containing the edge dislocation would appear dark, surrounded by bright intensity from the neighboring, undistorted crystalline material. In this case, contrast in the image would appear as a dark line across the bright-field image, since this dislocation line l i s parallel to the plane of the sample. This situation is illustrated by the bright-field image in Figure 5c, where a set of dislocations in shock-deformed N i f l is imaged. Each dislocation appears as a dark line on a bright background (each line appears paired in this image because these are dissociated superlattice dislocations). By comparison, Figure 5d is a dark-field image from the same region, which was obtained by placing the objective aperture around a diffracted beam in the SAD pattern instead of the transmitted beam. The same dislocations that were imaged in the bright-field mode in Figure 5c now appear as bright lines on a dark background. The dark background results because the undistorted crystal lattice is not well-aligned for diffraction, so little scattered intensity arises from these regions, to contribute brightness to this dark-field image. But the dislocations appear bright since diffracted intensity from the dislocation cores (that was lost in the bright-field mode) is now captured in the dark-field mode. This is typical of image contrast in the dark-field mode; consequently the name dark-field (i.e., bright objects on a dark background) is applied to this imaging technique. Dark-field microscopy is a powerful technique, but many associated subtleties complicate its practice. A most noteworthy example is the technique of weak-beam dark-field imaging5 The last exampie of imaging techniques in TEM is high-resolution transmission electron microscopy. High-resolution TEM is made possible by using a large-diameter objective diaphragm that admits not only the transmitted beam, but at least one diffracted beam as well. All of the beams passed by the objective aperture are then made to recombine in the image-forming process, in such a way that their amplitudes and phases are preserved. When viewed at high-magnification, it is possible to see contrast in the image in the form of periodic fringes. These fringes r e p resent direct resolution of the Bragg diffracting planes; the contrast is referred to as phase contrast. The fringes that are visible in the high-resolution image originate from those planes that are oriented as Bragg reflecting planes and that possess interplanar spacings greater than the lateral spatial resolution limits of the instrument. The principle here is the same as in the AbbC theory for scattering from grating in light optics.’ An example of an HRTEM image is shown in Figure 6. This image is of an epitaxial thin film of Y1Ba2Cu307-x grown on LAO3 (shown in cross section). The HRTEM technique has become popular in recent years due to the more common availability of high-voltage TEMs with spatial resolutions in excess of 112

IMAGING TECHNIQUES

Chapter 2

Figure 6

High-resolution transmission electron microscopy image of an epitaxial thin film of Y1Ba2Cu307-,grown on LaA103, shown in cross section. (Courtesy of T. E. Mitchell, Los Alamos National Laboratory)

0.2 nm. Image simulation techniques are necessary to determine the atomic structure of defects imaged by HRTEM. Specimen Preparation

Probably the most difficult, yet at the same time, most important aspect of the TEM technique is the preparation of high-quality thin foils for observation. This is an old, ever-expanding, complicated, and intricate field of both science and art. There is no simple way to treat this subject briefly. We will merely mention its importance and list some references for further details. It is important to realize (managers, take notice) that the most labor intensive aspect of TEM is the preparation of a useful sample. In the early days of TEM, sample preparation was divided into two categories, one for thin films and one for bulk materials. Thin-films, particularly metal layers, were often deposited on substrates and later removed by some sort of technique involving dissolution of the substrate. Bulk materials were cut and polished into thin slabs, which were then either electropolished (metals) or ion-milled (ceramics). The latter technique uses a focused ion beam (typically Ar+) of high-energy, which sputters the surface of the thinned slab. These techniques produce so-called planview thin foils. Today, there is great interest in a complementary specimen geometry for observation, that of the cross section. Cross sections usually are made of layered materi2.4

TEM

113

als. The specimens are prepared so as to be viewed along the plane of the layers. The techniques for producing high-quality cross sections are difficult, but rather well established. For additional information on sample preparation, consult Thompson-Russell and Edington' and the proceedings of two m osia on TEM sample ?, preparation sponsored by the Materials Research Society.

It

Conclusions

TEM is an established technique for examining the crystal structure and the microstructure of materials. It is used to study all varieties of solid materials: metals, ceramics, semiconductors, polymers, and composites. With the common availability of high-voltage TEM instruments today, a growing emphasis is being placed on atomic resolution imaging. Future trends include the use of ultrahigh vacuum TEM instruments for surface studies and computerized data acquisition for quantitative image analysis.

Related Articles in the Encyclopedia

STEM, SEM, EDS, EELS

References 1

M. von Heimendahl. Electron Microscopy ofMaterialc An Introduction. Materials Science and Technology Series (A. S. Nowick, ed.) Academic, New York, 1980, Chapter 1.This is an excellent introductory guide to the principles ofTEM.

2

L. Reimer. TransmissionElectron Microscopy: Physics ofImage Formation and Microanalysis. Springer-Verlag, Berlin, 1984. This is an advanced but comprehensive source on TEM. Reimer also authored a companion volume on SEM. I? B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley, and M. J. Whelan. Electron Microscopy of Thin Cvstah. Buttenvorth, Washington, 1965, Chapter 4. This sometimes incomprehensible volume is the classic textbook in the field ofTEM. R. H. Geiss. Introductory Electron Optics. In: Introduction to Analytical Ekctron Microscojy. (J. J. Hren, J. L. Goldsrein, and D. C. Joy, eds.) Plenum, New York, 1979, pp. 43-82. J. W. Edington. Practical Ehctron Microscopy in MateriaLr. van Nostrand Reinhold, New York, 1976. This is an excellent general reference and laboratory handbook for the TEM user.

3

4

5

114

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B.D. Cdity. Elements of X-RayDzfiaction. Addison-Wesley, Reading, 1956.This is a good general reference concerningX-ray diffraction techniques. 7 E. Hecht and A. Zajac. Optics. Addison-Wesley, Reading, 1974, Chapter 14. This entire book is an invaluable reference on the principles of optics. 8 K. C. Thompson-Russell and J. W. Edington. Electron Microscope Specimen Preparation Tecbniques in Materiah Science. Monographs in h m > a I Electon Microscopj No. 5. Philips Technical Library, Eindhoven & Delaware, 1977. 9 Specimen Pwparationfir TransmissionEkctron Microscopy I . (J. C. Bravman, R. M. Anderson, and M. L. McDonald, eds.) Volume 115 in MRS symposium proceedings series, 1988. i o Specimen Preparationf i r TrammissionEkctron Microscopy II. (RM. Anderson, ed.) Volume 199 in MRS symposium proceedings series, 1990. 6

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ELECTRON B E A M INSTRUMENTS 3.1 Energy-Dispersive X-Ray Spectroscopy, EDS 120 3.2 Electron Energy-Loss Spectroscopyin the Transmission Electron Microscope, EELS 135 3.3 Cathodoluminescence,CL 149 3.4

ScanningTransmission Electron Microscopy, STEM

161

3.5 Electron Probe X-Ray Mircoanalysis, EPMA 175

3.0 INTRODUCTION Whereas the previous chapter emphasizes imaging using microscopes, this chapter is concerned with analysis (compositional in particular) using fine electron probes: which provide fine spatial resolution. The beams used are either those in the SEM and TEM, discussed in the previous chapter (in which case the analytical techniques described here are used as adjuncts to the imaging capabilities of those instruments), or they involve electron beam columns specially constructed for an analytical mode. The Scanning Transmission Electron Microscope, STEM, and the Electron Microprobe, used for Electron Probe Microanalysis, EPMA, are two examples of the latter that are discussed in this chapter. A third example would be the Auger electron microprobe, used for scanning Auger Electron Spectroscopy, AES, but we choose to discuss this technique in Chapter 5 along with the ocher major electron spectroscopy methods, since all of them are primarily used to study true surface phenomena (monolayers), which is not generally the case for the techniques in this chapter. The incoming electron beam interacts with 'the sample to produce a number of signals that are subsequently detectable and useful for analysis. They are X-ray emission, which can be detected either by Energy Dispersive Spectroscopy,EDS, or by Wavelength Dispersive Spectroscopy, WDS; visible or W emission, which is known as Cathodoluminescence, CL; and Auger Electron Emission, which is the basis ofAuger Electron Spectroscopydiscussed in Chapter 5. Finally, the incoming 117

beam itself can lose discrete amounts of energy by inelastic collision, the values of which are determined by an electron energy analyzer. This is the basis of Electron Energy Loss Spectroscopy, EELS. Which of these classes of processes is more dominant, or more useful, depends on a number of factors, including the energy of the electron beam, the nature of the material (high or low Z), and the type of information sought (elemental composition, chemical composition, ultimate in spatial resolution, information limited to the surface, or information throughout the bulk by transmission measurement). A complete perspective for this can be obtained by comparing the articles in this section, plus the AES article, since they interrelate quite strongly. Some brief guidelines are given here. All the methods, with the exception of CL, provide elemental composition. The most widely used is X-ray emission. If EDS is used the package can be quite inexpensive ($25,000 and up), and can be routinely fitted to SEMs, TEMs, and STEMS.In addition EDS is one of the two detection schemes in EPMA (the other is WDS). Its great advantage is its ability to routinely provide rapid multi element analysis for A l l , with a detection limit of about 200 ppm for elements with resolved peaks. Its major disadvantages are very poor energy resolution, so that peaks are often overlapped; a detector problem that adversely affects detection limits; and the fact that the detector must remain cooled by liquid nitrogen or risk being destroyed. All these shortcomings of the EDS detector can be overcome by using the other detection scheme, WDS. The disadvantages of this scheme are that it is more expensive and cumbersome experimentally and does not have simultaneous multi element detection capability. For these reasons it is not so much used in conjunction with an SEM, TEM, or STEM, but is the heart of the ehctron microprobe, which is designed to combine WDS and EDS in the most effective analytical way. The spatial resolution of X-ray emission does not usually depend on the diameter of the electron beam, since small beams spread out into a roughly pear-shaped “interaction volume” below the sample surface, and it is from this region that the X-ray signal is generated. This volume varies from a fraction of a micron to several microns depending on the electron beam energy (lower energy, smaller volume), and the material (lower 2, smaller volume). The exceptions are when the beam width is larger than a few microns, in which case it starts to dominate the resolution, or when the sample is very thin (hundreds of angstroms or less) so that the beam passes through before it can spread much. In this case the spatial resolution can be greatly improved toward that of the beam size itself. This is the case for thin samples in a TEM or STEM. Cathodoluminescence,CL, involves emission in the UV and visible region and as such is not element specific, since the valence/conduction band electrons are involved in the process. It is therefore sensitive to electronic structure effects and is sensitive to defects, dopants, etc., in electronic materials. Its major use is to map out such regions spatially, using a photomultiplier to detect all emitted light without 118

ELECTRON BEAM INSTRUMENTS

Chapter 3

spectral resolution in an SEM or STEM. Spatial resolution and depth resolution capabilities are, in principle, similar to X-ray emission, since the UV/visible emission comes from roughly the same interaction region. In practice lower electron beam energies are sometimes used in CL to improve spatial resolution. EELS is used in a transmission mode in conjunction with TEMs and STEMS. Samples must be very thin (hundreds of angstroms) and beam energies must be high (100 keV and up) to prevent the single scattered EELS signal from being swamped by a multiple scattering background. A direct consequence of this requirement is that the spatial resolution of transmission EELS is not much worse than the beam size, since a 100-kV electron passing through a sample and scattered only once does not deviate much in direction. Thus, in a STEM with a 2-Abeam size the spatial resolution of EELS for a sample 100 A thick might be only 3-4 Although the main use of transmission EELS is to provide elemental composition like EDS/WDS it can also provide much information about chemical states and about electronic structure from the line shapes and exact positions of the energy loss peaks. EELS is also used in a reflection mode (REELS) in Auger spectrometers for surface analysis (see Chapter 5). The STEM instrument itself can produce highly focused high-intensity b-Gams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneouscrystallographicinformation can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over $1,000,000. Electron Probe Microanalysis, EPMA, as performed in an ekctron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components ( Z > 1l), several hours for minor components, and about 10 hours for trace elements.

119

3.1

EDS Energy-DispersiveX-Ray Spectroscopy R O Y H. G E I S S

Contents Introduction Principles of X-Ray Production Instrumentation Resolution, Peak Overlap, and Minimum Detection Sample Requirements Operational Considerations in Electron Microscopes Quantification Conclusions

Introduction

With modern detectors and electronics most Energy-Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, 2 = 4,if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 x 105 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1-2% wt. For elements with Z c 10, the MDL is usually around 1-2% wt. under the best conditions, especially in electron-beam instruments. The accuracy of quantitative analysis has been reported to be better than 2% relative for major concentrations, using well-polished standards having a composition similar to the sample. A more conservative figure of 4-5% relative should be expected for general analysis using pure element standards. For analysis without 120


Chapter 3

using standards the accuracy will usually be much worse. The analysis of elements with concentrations less than 5% wt. will typically yield relative accuracies nearer lo%, even with standards. For samples with rough surfaces, such as fracture samples or small particles, the relative accuracy may be as bad as 50%. Most applications of EDS are in electron column instruments like the scanning electron microscope (SEM), the electron probe microanalyzer (EPMA), and transmission electron microscopes (TEM). TEMs are hrther classified as conventional transmission (CTEM) or scanning transmission (STEM) instruments, depending on whether scanning is the primary imaging mode. A CTEM equipped with a scanning attachment and an EDS instrument is an Analytical Electron Microscope (AEM). X-ray spectrometers, with X-ray tube generators as sources and Si (Li) detectors have been used for both X-Ray Fluorescence Spectroscopy (XRF) and XRay Diffraction (XRD). Portable EDS systems also have been constructed using Xray tube generators or radioactive sources. A spectrum can be obtained from almost any sample, as long as it can be put on the specimen stage of the microscope. The choice of acceleratingvoltage should be determined by the type of sample one is studying, since the X-ray generation volume depends on the electron range in the material. In the study of thin films it is usually desirable to minimize the electron range and use an acceleratingvoltage just greater than &, the critical ionization voltage for the X-ray line of interest. For bulk samples it is more important to maximize X-ray production regardless of the electron range and, as will be discussed later, the acceleratingvoltage should be ideally 2-2.5 x 4.For example, consider the K-shell ionization of copper, for which E, = 8.98 keV. To analyze a film only a few nm thick on a Si substrate, using the copper Ka, the accelerating voltage should be set near 10 keV. To analyze a bulk sample, more than a f i x prn thick, an acceleratingvoltage of 20-25 keV should be used. With an MDL of 100-200 ppm for most elements, an EDS system is capable of detecting less than a monolayer of metal film on a substrate using K a lines at moderate acceleratingvoltages of 5-1 5 keV. Since many SEMs now have field emission electron guns providing high brightness probes at voltages of 2 keV and less, EDS analysis of even thinner films should be possible, at least in principle, since the electron range and hence, the generated X-ray volume will be very small. In this case, however, since all the X-ray lines will be low energy and in a small energy region, there may be many overlapped peaks that will have to be deconvoluted before quantitative analysis can be attempted. This deconvolution can be tricky, however, since the shape of the background in this energy range is difficult to model. In addicion, the shape of the peaks in the low-energy region is often not Gaussian and the peak positions, especially for the K lines fiom low-Zelements, are often shifted. Energy-dispersive X-ray spectroscopy has been used for quality control and test analysis in many industries including: computers, semiconductors, metals, cement, paper, and polymers. EDS has been used in medicine in the analysis of blood, tis3.1

EDS

121

sues, bones, and organs; in pollution control, for asbestos identification; in field studies including ore prospecting, archeology, and oceanography; for identification and forgery detection in the fine arts; and for forensic analysis in law enforcement. With a radioactive source, an EDS system is easily portable and can be used in the field more easily than most other spectroscopy techniques. The main advantages of EDS are its speed of data collection; the detector’s e%ciency (both analytical and geometrical);the ease of use; its portability; and the relative ease of interfacing to existing equipment. The disadvantages are: poor energy resolution of the peaks, (a typical EDS peak is about lOOx the natural peak width, limited by the statistics of electron-hole pair production and electronic noise, which often leads to severe peak overlaps); a relatively low peak-to-background ratio in electron-beam instruments due to the high background coming from bremsstrahlung radiation emitted by electrons suffering deceleration on scattering by atoms; and a limit on the input signal rate because of pulse processing requirements. Principlesof X-Ray Production

X-rays are produced as a result of the ionization of an atom by high-energy radiation wherein an inner shell electron is removed. T o return the ionized atom to its ground state, an electron from a higher energy outer shell fills the vacant inner shell and, in the process, releases an amount of energy equal to the potential energy difference between the two shells. This excess energy, which is unique for every atomic transition, will be emitted by the atom either as an X-ray photon or will be selfabsorbed and emitted as an Auger electron. For example, if the K shell is ionized and the ejected K-shell electron is replaced by an electron from the L, shell, the emitted X ray is labeled a characteristic Ka, X ray. (See Figure 2 in the article on electron probe X-ray microanalysis). The hole that exists in the L shell will be filled by an electron from a higher shell, say the M shell, if one exists. This M-L transition may result in the emission of another X ray, labeled in turn according to one of the many M-L transitions possible. The cascade of transitions will continue until the last shell is reached. Thus, in an atom with many shells, many emissions can result from a single primary ionization. Instrumentation

The heart of the energy-dispersive spectrometer is a diode made from a silicon crystal with lithium atoms diffused, or dnped from one end into the matrix. The lithium atoms are used to compensate the relatively low concentration of grown-in impurity atoms by neutralizing them. In the diffusion process, the central core of the silicon will become intrinsic, but the end away from the lithium will remain ptype and the lithium end will be n-type. The result is a p-i-n diode. (Both lithium122


Chapter 3

Liquid

ei Figure 1

Nitrogen Dewar

Schematic of an EDS system on an electron column. The incident electron interacts with the specimen with the emission of X rays. These X rays pass through the window protecting the Si (Li) and are absorbed by the detector crystal. The X-ray energy is transferred to the Si (Li) and processed into a digital signal that is displayed as a histogram of number of photons versus energy.

drifted and pure intrinsic germanium have been used as detectors, but much less frequently. These will be discussed later). A reverse bias electrical field of 1001000 volts is applied to thin layers of gold evaporated onto the front and back surh s of the diode. When an X-ray photon enters the intrinsic region of the detector through the ptype end, there is a high probability that it will ionize a silicon atom through the photoelectric effect. This results in an X ray or an Auger electron, which in turn produces a number of electron-hole pairs in the Si (Li): one pair per 3.8 eV of energy. For example, a 6.4-keV X ray absorbed by the silicon atoms will produce about 1684 electron-hole pairs or a charge of about 2.7 x Coulombs. Both charge carriers move freely through the lattice and are drawn to the detector contacts under the action of the applied bias field to produce a signal at the gate of a specially designed field effect transistor mounted directly behind the detector crystal. The transistor forms the input stage of a low-noise charge-sensitive preamplifier located on the detector housing. The output from the preamplifier is fed to the main amplifier, where the signal is finally amplified to a level that can be processed by the analog-to-digital converter (ADC) of the multichannel analyzer ( M U ) . See Figure 1. The height of the amplifier output pulse is proportional to the input preamplifier pulse, and hence is proportional to the X-ray energy. For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically 3.1

EDS

123

9000-

Si-Ka

8Ooo-

7000-

Bi-Ma Fe-Ka ? .a

3Ooo-

2000-

Bi-La

IOOOI

I

I

8 10 12 Energy (kev) Figure 2

I

14

16

18

20

Standard output of an EDS spectrum. The horizontalaxis is the energy scale is and the vertical axis is the number of photons per energy interval. The X-ray identification, element and line, is indicated in the vicinity of the peaks.

operating at 50 MHz. The time to discharge the capacitor to 0 V is proportional to the pulse amplitude, and hence to the X-ray energy. The 50-MHz clock produces a binary number in one of the 1024 channels typically used by the MCA in accordance with the time of the discharge, and increments the previously collected number in that channel by 1. By an energy calibration of the channels in the M U , the collection of X-ray pulses may be displayed on a CRT as an energy histogram. A typical spectrum output from a thin alloy film using a TEM is shown in Figure 2. T o partition the incoming X rays into their proper energy channels, it is necessary to measure only single pulses. At high count rates, however, situations often arise where a second pulse reaches the main amplifier during the rise time of the preceding pulse. The two pulses may then combined into a single pulse whose energy is the combined energy of the two individual pulses. This process is known as pulse pile-up and the output, which is an artifact, is called a sum peak. Pile-up effects can be minimized with the use of electronic pulse rejection using a second pulse amplifier with a much faster response. Operationally, it is usually desirable to collect spectra with a dead time of less than 40%, or at an input count rate of about 5000 cps. 124


Chapter 3

Detectors are maintained under vacuum at liquid nitrogen temperature to reduce electronic noise and to inhibit diffusion of the lithium when the bias voltage is applied. Most have a window covering the entrance to provide vacuum protection. The window in a standard detector is usually made f;om rolled beryllium foil, a few pm thick. Unfortunately, low-energy, or mfi, X rays are strongly absorbed by the Be window, limiting the analysis range of these detectors to elements having atomic number Z > 10. To reduce the absorption, detectors are built either without windows at all or with windows made of new low-Zmaterials that can withstand atmospheric pressure. Detectors made with these low-Z windows show a marked increase in sensitivity to X rays from elements having Z e 10, compared to the Be window detectors. Detector sensitivity drops off at the high-energy end (greater than 20 kev), where the Si (Li)crystal, which is typically 3-5 mm thick, becomes transparent to high-energy X rays. This may not be important in an SEM operated at 30 keV, since high-energy X rays will not be excited. But in a TEM operated at 100 keV or higher, the ability to detect hard X rays would be severely limited. In Figure 3, curves are plotted that show the detector's efficiencyboth at low energy, where the X-ray absorption of a standard Be window is compared to that of a low-Zwindow, and at high energy, where the high-energy drop-off is shown for the standard thickness Si (Li) crystals used today. Spectrometers have been made using germanium crystals. The Ge crystals have been either lithium drified, Ge (Li), or more recently, made from high-purity intrinsic germanium, HPGe. The HPGe crystal has the advantage that it can be allowed to warm to room temperature if it is not being used, saving the hassle of keeping the liquid nitrogen dewar filled. Germanium is much less transparent to high-energyelectronsthan silicon, because Ge (Z= 32) has a higher stopping power than Si (Z= 14), and should be able to detect very high energy X rays such as gold Kayat 69 keV. This will be a distinct advantage when a TEM, operatingat 100 keV and above and therdore capable of exciting high-energy X-ray lines, is used in the study of alloys containing elements that have severe peak overlap in the lower energy lines. However, germanium has some drawbacks; namely, a K-shell absorption edge at 11.1 keV, a complex L-shell absorption' edge structure starting at 1.4 keV and a series of escape peaks in the range 2-12 keV. Thus, in the energy range 1-10 keV, the most frequentlyused range in EDS analysis, the detector is not as well behaved as Si (Li). A major advantage of the energy-dispersive spectrometer is that it can be positioned very close to the sample and can present a large solid angle for the collection of emitted X rays. The solid angle in a typical EDS configuration is about 10 times greater than that of a WDS. With EDS, more X-rayphotons will be collected per incident electron, so that either a smaller probe diameter or lower beam current can be used (which reduces specimen damage). Detectors are usually mandactured with an active area of either 10 mm2 or 30 mm2 diameter. 3.1

EDS

125

X-Ray hergy (keV)

Figure 3

Curves showing the absorption of the window materials at low energy for a standard Be window and a low-Zwindow. The high-energy region shows the transmission of X rays through 3 mm and 5 mm thick Si (Li) crystals. Most detectors can be represented by a combination of one of the low-energy curves and one of the high-energy curves.

Since energy-dispersive spectrometers consist mostly of electronic components, they are easy to interface to most instruments. The only limitations are the need for a large liquid nitrogen cryostat to cool the spectrometer and high vacuum for the windowless detectors. Some Si (Li) detectors use mechanical cooling, called Peltier cooling, instead of the liquid nitrogen; this eliminates the large cryostat and the nagging requirement to keep it full. Unfortunately, the temperature reached with Peltier refrigerators is not as low as that obtained with liquid nitrogen, and the detector resolution suffers. Most EDS systems are controlled by minicomputers or microcomputers and are easy to use for the basic operations of spectrum collection and peak identification, even for the computer illiterate. However, the use of advanced analysis techniques, including deconvolution of overlapped peaks, background subtraction, and quantitative analysis will require some extra training, which usually is provided at installation or available at special schools. 126


Chapter 3

Resolution, Peak Overlap, and Minimum Detection

The energy resolution of a solid state detector is defined as the full width at half maximum (FWHM) of the peak obtained at energy 4 when a monoenergetic beam of X rays having energy E, is incident on the detector. Ideally this width would be very small; but due to the statistical nature in the collection process of the electron-hole pairs, there will be some fluctuation in the measured energy of the X rays. For a modern energy-dispersive spectrometer, the FWHM of the peaks ranges from about 70 eV for the C Kor at 0.282 keV to about 150 eV for the Cu Ka at 8.04 keV. Improvements in detector resolution have focused on reducing the electronic noise contribution by improving the FET design and by using nonoptical charge restoration. Noise resolution as low as 40 eV has been achieved, giving a detector resolution of 128 eV FWHM at 5.89 keV. Peak overlap, which follows from the poor resolution of the detector, is one of the major problems in EDS. For example, severe peak overlap occurs when two peaks of about the same amplitude are separated by less than half of the FWHM of the peaks. In this instance they will merge together and appear as one Gaussian peak. Unfortunately, in X-ray spectroscopy the peaks of the characteristic lines are often closer together then the resolution of the best EDS systems, and significant peak overlap follows. This presents problems not only with the identification of the individual peaks, but also with the determination of the amplitudes of the peaks for quantification. Improvements in detector resolution will obviously provide some relief to the problem, but even for the most optimistic resolution specification, there will still be many instances of peak overlap. An example of this is shown for the BaTiO, system in Figure 4, where the spectrum from a standard EDS is compared to that obtainable with a standard WDS using a LiF crystal; in the latter case the FWHM of the peaks is only a fav eV. Peak overlap presents a particularly difficult problem in the analysis of soft X rays, especially when the K-line X rays from elements with Z< 10, the L-line X rays from transition metals or the M-line X rays from the rare earths are present. As previously mentioned, the resolution of most EDS detectors is about 60-90 eV in this region, which ranges from 300 to 1000 eV. To compound the problem, the peak shape is usually not purely Gaussian in this region, making computer deconvolution even more difficult. Here again, the resolution of a wavelength spectrometer will be only 5-10 eV, so that with the proper choice of analyzing crystal, peak overlap will not be a problem. The minimum detection limit, MDL, of an isolated peak on a uniform background is proportional to the square root of the FWHM. So a 20% reduction in Spectrometer resolution will produce about a 10% improvement in MDL. If there is peak overlap, however, then it can be shown that a 20% improvement in resolution can reduce the interference between overlapping peaks by a factor of 3, which gives about a 50% improvement in MDL. 3.1

EDS

127

'4-.

Figure4

4.2 -..&

w 4:4 7.7

4.6 -..6

I

4.8

5

5.2

514 5.4

516 5.6

5:8 5.8

6

Superposed EDS and WDS spectra from BaTiO,. The EDS spectrum was obtained with a detector having 135-eV resolution, and shows the strongly overlapped Ba La,-Ti K a and Ba Lp,-Ti Kp peaks. The WDS spectrum from the same material shows the peaks to be completely resolved.

The MDL, or trace element sensitivity, is inversely proportional to P2/B, the ratio of the peak counting rate and the background counting rate for a pure element. If data is collected on both EDS and WDS instruments using the low probe current appropriate to EDS (a few nA) the MDL with EDS will be 3-5x better than that obtained with W D S . But if WDS is used at its normally higher probe currents (2&100 nA) the MDL obtained using WDS will be about lox better! The major obstacle to a lower MDL in EDS, aside from the peak overlap problem, is a much higher background (due to bremsstrahlung radiation) and therefore a much lower P2/Bratio relative to WDS. In cases where the peaks are not overlapping it should be possible to detect and analyze 0.1-0.2 nm (about one monolayer) of a metal frlm on a conducting substrate using EDS. Sample Requirements

Almost any size solid sample can be studied in the SEM or EPMA. The only limitation is the size of the specimen chamber, which is usually at least 10 cm in diameter. In a standard SEM, however, the x-y translation is usually limited to 25 mm. In SEMs with an air lock for sample exchange, the maximum sample size may be only 3-5 cm in diameter. A highly polished surface is required for accurate quanti128


Chapter 3

tative analysis, since surface roughness will cause undue absorption of the generated X-ray signal, which is difficult to account for in the quantification procedure. Powder samples can be compacted or fixed in epoxy and analyzed in the SEM or EPMA. However, with the high vacuum required in the electron column, liquids are not easily studied. In the TEM, the sample must be transparent to the incident electrons and therefore may be only a few hundred nm thick. The maximum sample diameter is usually 3 mm. In electron column instruments, nonconducting samples usually present a problem due to excess charge build-up. To provide a conducting path to ground they are coated with a thin conducting film of C or Al, which must be accounted for in the final analysis if the absorption by the coating material is significant. For EDS combined with an X-ray tube generator or radioactive source in air, most of these restrictions are relaxed and almost any sample can be analyzed. Coating is not required for nonconducting specimens. As mentioned before, samples with rough surfaces, such as fracture samples, may give a spectrum, but the relative accuracy of the quantitative analysis will not be very good.

Operational Considerations in Electron Microscopes

For inner shell ionization to take place, the energy of the incident radiation must be greater than the ionization energy of the particular atomic shell in question. For electrons, the ionization cross section Q is calculated using a hnction of the form (11U)ln U,where U=&/E;, is the ratio of the energy of the incident electron 4 to the critical ionization energy 4. Qualitative evaluation of this hnction shows that Q reaches a maximum around U=2.5, but is almost constant for U >2. To a first approximation, therefore, the only requirement is that the accelerating voltage of the electron gun be chosen such that U >2. In the analysis of trace elements or thin films on substrate using electrons, however, one finds that the MDL, may be increased by choosing E, such that Uis just greater than 1. The reason for this is that the k factor, which is the ratio of the intensity from the sample to that from the standard, increases as Uapproaches 1 for thin films. Thus, by maximizing the k factor, the sensitivity is increased. For bulk sample analysis, however, the k factor will usually be a maximum at U - 2.5. Another consideration in the determination of the optimum 4 is the depth of X-ray production in bulk samples, especially if one component strongly absorbs the radiation emitted by another. This is often the case when there is a low-Zelement in a high-Zmatrix, e.g., C in Fe. Here X rays from carbon generated deep within the sample will be highly absorbed by the Fe and will not exit the sample to be detected. The usual result will be an erroneously low value for the carbon concentration. In these situations the best choice for E, will be closer to & with U = I. rather than a much higher value with U =2.5. 3.1

EDS

4 29

K-O

X-rayline

CKa

SiKa

GKa

CuKa

AuLa

E, (kev)

0.283

1.838

5.988

8.980

11.919

Si

4.737

4.653

4.108

3.496

2.743

4.741

Cr

1.760

1.729

1.526

1.299

1.019

1.761

Ag

1.376

1.351

1.193

1.015

0.797

1.377

AU

0.861

0.846

0.747

0.636

0.499

0.862

*

1 A90

ra”ge

M a 6 element

Range (pm)for & = 10 keV

Si

1.486

1.402

0.857

0.245

Cr

0.552

0.521

0.318

0.091

0.554

Ag

0.432

0.407

0.249

0.071

0.433

AU

0.27

0.255

0.156

0.045

*

0.271

=.

= no X rays generated.

Table 1

Range of X-ray generation calculated by the Kanaya-Okayama (K-O) equation for selected X-ray lines in low- to high-density materialsfor two incident elecFor comparison, the range of electrons with E, = 0, is given tron energies, is the incident electron energy, E, is the critical in the last column. (Here ionization energy, A is the atomic weight, Zis the atomic number, and p is the density.) The K-O X-ray range is given by the equation: R= 0.0276 A(€O1.mE,’-~)/ (20.889p) pm.

Range and Lateral Resolution of X-Ray Generation

The range over which characteristicX rays are generated depends both on the properties of the sample and the choice of accelerating voltage. X-ray generation ranges calculated for low-, medium- and high-density materials under a few electronaccelerating voltages are given in Table 1 to illustrate this point. For comparison, the electron range is included in the table. As can be seen, the X-ray generation range is always less than the electron range because characteristic X rays are generated only when the electron has energy greater than E,. But as can be seen in the table, the difference between the X-ray range and the electron range is very significant only when E, is nearly equal to 4.Accordingly, the X-ray generation volume is smaller than the electron interaction volume and the volume from which the X rays are measured is even smaller because of X-ray absorption. (Another discussion of this topic can be found in the article on EPMA.) 130


Chapter 3

It also should be recognized that the density of X-ray production is not constant throughout the X-ray generation volume, but is related to the number and length of electron trajectories per unit volume and the average overvoltage along each trajectory. The distribution of the generated X rays as a function of the mass thickness, or depth, in the sample is given by a mathematical expression that has the shape of a Gaussian curve offset at the surface of the sample. This function is often called the $(pz) hnction. The lateral resolution of X-ray analysis in an SEM or EPMA is not generally related to the incident probe size. From the values for the X-ray generation ranges given in Table 1 one can see that the diameter of the X-ray generation volume in thick samples is much greater than a fay tens of nm, which would be the nominal diameter of a medium-resolution probe in an SEM. On the other hand, with a thin-foil sample in a TEM, the probe size will be very important in the determining the lateral resolution, since most of the incident electrons pass through the foil with only a fay collisions and therefore are not be deflected fir from their incident direction.

Modes of Analysis

There are three modes of analysis commonly used: spectrum acquisition; spatial distribution, or dot, mapping of the elements; and elemental line scans. In the spectrum acquisition mode the probe is either fEed in the spot mode or raster scanned over a small area at high magnification and a complete spectrum acquired. A typical spectrum is shown in Figure 2. In an electron microscope or microprobe with scanning capability, data from more than 15 elements can be collected simultaneouslyand used to generate X-ray dot maps displaying the spatial distribution of the elements in the sample. In this technique the brightness of the CRT is modulated as the beam is raster scanned to reflect the X-ray output of the element of interest, which usually is designated by defining a region of interest (ROI) around the peaks in the EDS spectrum at that point. Some analyzers provide the capability to remove the background from the signal before plotting. Multielement maps can be made with a color CRT, using a different color chosen for each element. These maps can be arranged in an overlay with the colors combined to show the presence of various compounds. A long data collection time, sometimes hours, is often required to collect enough data points for a high-resolution dot map. With the same scanning capability, it is much faster and often more usell to simply scan one line on a sample. The data is again output to a color CRT, but it is presented as the modulation of the yamplitude, which is determined by the intensity of the X-ray signal production from the ROI of the element of interest. As the probe scans along the line, the CRT plots a graph of the elemental counting rates versus distance. Here again, it is usually possible to plot the data from many ele3.1

EDS

131

ments simultaneously.This mode of display will usually show minor concentration differences much better than the brightness modulation technique used in mapping. Quantification

The ultimate goal in using an EDS analytical system is to be able to measure the concentrations of all the elements in the sample. To this end, a series of measurements are made in which the peak intensity from each element in the sample is compared to the peak intensity obtained from a reference standard using the same operating conditions. The ratio of these two intensities is the k factor mentioned previously. To convert the measured k factors to the composition, it is usually necessary to make corrections that account for deviations from a linear relationship between the k factor and the composition. These include an atomic.number correction accounting for the fraction of electrons backscattered by the sample and the volume of X-ray generation (the Zfactor); a correction for the absorption of the generated X rays in the sample (the A factor); and a correction for secondary X-ray fluorescence in the sample (the Ffactor). Various procedures have been developed to do this; the most common is known as the ZAF correction, with C(%)= W . In another approach, which was previously mentioned, the mass thickness, or depth distribution of characteristicX-ray generation and the subsequent absorption are calculated using models developed from experimental data into a $(pz) function. Secondary fluorescence is corrected using the same Fhctors as in ZAF. The @(pz)formularion is very flexible and allows for multiple boundary conditions to be included easily. It has been used successfully in the study of thin films on substrates and for multilayer thin films.

Conclusions

EDS is an extrepely powerful analytical technique of special value in conjunction with electron column instruments. In a few seconds a qualitative survey of the elements present in almost any sample can be made, and in only a few minutes sufficient data can be collected for quantification. The most frequent application has been to highly polished metal samples, although nonconducting samples may be studied if they can be coated with a thin conducting film. Sample dimensions are usually not a problem in an SEM. Most of the detectors used to date have been of the Si (Li) type, but with improvements in the processing methods of germanium, HPGe detectors may start impacting the market. New cooling methods, e.g., Peltier cooling, are being studied and will be a significant improvement if the ultimate temperature can be lowered to nearer to liquid nitrogen temperatures. Detector resolution is still the most serious problem. It is slowly being reduced through improvements in FET design and new methods of charge restoration. New window 132


Chapter 3

materials have made low-2 detectors the standard, opening up the prospects of light element analysis. However, the sensitivity for light elements is generally low and the analytical methods are still being developed because the peak shapes are often not Gaussian and the mass absorption coefficientsfor the low-Zelements are not very well known. Overall a “customer” needs to know under what circumstances it is best to use either the electron-beam techniques of EDS and WDS or the X-ray technique of XRF for an analysis problem. If both are equally available, the choice usually resides in whether high spatial resolution is needed, as would be obtained only with electron-beam techniques. If liquids are to be analyzed, the only viable choice is XRF. If one’s choice is to use elearon-beam methods, the h t h e r decision between EDS and WDS is usually one of operator preference. That is, to commence study on a totally new sample most electron-beam operatorsw ill run an EDS spectrum first. If there are no serious peak overlap problems, then EDS may be sufficient. If there is peak overlap or if maximum sensitivity is desired, then WDS is usually preferred. Factored into all of this must be the beam sensitivity of the sample, since for WDS analysis the beam current required is 10-1OOx greater than for EDS. This is of special concern in the analysis of polymer materials. Related Articles in the Encydopedia

EPMA, STEM, TEM, SEM, XRF Bibliography 1

2

3

3.1

R. Woldseth. X-RayEnergy Specmrneq Kevex Corporation, San Carlos, 1973. A good introduction with emphasis on detectors and electronics. Most of the applications refer to X-ray tube sources. Unfortunately the book is out of print, but many industrial laboratories may have copies. Quuntitative Electran-Probe MimanaLysh.(VD. Scott and G. Love, eds.) John Wiley & Sons, New York, 1983. Taken from a short course on the electron microprobe for scientists working in the field. A thorough discussion of EDS and WDS is given, including experimental conditions and specimen requirements. The ZAF correction fictors are treated extensively, and statistics, computer programs and Monte Carlo methods are explained in detail. Generally, a very useful book. J. I. Goldstein, D. E. Newbury, I?Echlin, D. C. Joy, C. Fiori, and E. Lifshin. ScanningELectron Microscopy andX-Ray Ana&s. Plenum Press, New York, 1981. Developed from a short course held annually at Lehigh Universiy The book is concerned with the use and applications of SEM. In the latter context a lengthy discussion of EDS is given. The discussion EDS

133

on analysis is not as thorough as that in Scott and Love, but serves as an 4

5

excellent introduction, with all the important topics discussed. D. B.Williams. PrmticalAnalyticalEkctron Microscopy in Materialj Science. Verlag Chemie International, Weinheim, 1984. A good monograph discussing the use and applications of AEM, especially at intermediate voltages. The discussion on EDS is an excellent primer for using X-ray analysis on a TEM. Principles OfAnalytical Ehctron Microscopy. (D. C . Joy, A. D. Romig, and J. I. Goldstein, eds.) Plenum Press, New York, 1986. Another book, more readily available, discussing all aspects of AEM. Approximately one-quarter of the book is devoted to EDS and a discussion of thin-film analysis in the TEM.

134


Chapter 3

3.2

EELS Electron Energy-Loss Spectroscopy in the Transmission Electron Microscope NESTOR J . ZALUZEC

Contents

9

Introduction Basic Principles Low-Loss Spectroscopy Inner Shell Spectroscopy Quantification Limitations and Specimen Requirements Conclusions

Introduction

Electron Energy-Loss Spectroscopy (EELS) is an analytical methodology which derives its information from the measurement of changes in the energy and angular distribution of an initially nominally monoenergetic beam of electrons that has been scattered during transmission through a thin specimen. This geometry is identical to that which is used routinely in a transmission or Scanning Transmission Electron Microscope (TEM or STEM), and thus in the last decade the technique has been closely associated with these two types of instruments. EELS is an absorption spectroscopy and is similar in many respects to X-Ray Absorption Spectroscopy (XAS). Its characteristic spectral signature, termed the edge profile, is derived from the excitation of discrete inner shell levels to empty states above the Fermi level. Conservation of energy requires that the incident electron beam lose the corresponding amount of energy and intensity expended in exciting each inner shell level, and these are the parameters which are recorded in each experiment. From an analysis of the transmitted electron intensity distribution, the experiment can 3.2

EELS

135

derive the local elemental concentration of each atomic species present. Additionally, by studying the detailed shape of the spectral profiles measured in EELS, the analyst may derive information about the electronic structure, chemical bonding, and average nearest neighbor distances for each atomic species detected. A related variation of EELS is Reflection Electron Energy-Loss Spectroscopy (REELS). In REELS the energy distribution of electrons scattered from the surface of a specimen is studied. Generally REELS deals with low-energy electrons (e 10 kev), while TEM/STEM-based EELS deals with incident electrons having energies of 100400 keV. In this article we shall consider only the transmission case. REELS is discussed in Chapter 5. In principle, EELS can be used to study all the elements in the periodic table; however, the study of hydrogen and helium is successful only in special cases where their signals are not masked by other features in the spectrum. As a matter of experimental practicality, the inner shell excitations studied are those having binding energies less than about 3 keV. Quantitative concentration determinations can be obtained for the elements 3 5 Z I 35 using a standardless data analysis procedure. In this range of elements, the accuracy varies but can be expected to be +10-20% at. By using standards the accuracy can be improved to +1-2% at. Detection limit capabilities have improved over the last decade from 10-l' g to g. These advances have arisen through improved instrumentation and a more complete understanding of the specimen requirements and limitations. The energy resolution of the technique is limited today by the inherent energy spread of the electron source used in the microscope. Conventional thermionic guns typically exhibit an energy spread of 2-3 eV, and LaB6 a spread of about 1-2 eV; field emission sources operate routinely in the 0.25-1 eV range. In all cases, the sample examined must be extremely thin (typically < 2000 A) to minimize the adverse effects of multiple inelastic scattering, which can, in the worse cases, obscure all characteristic information. The uniqueness and desirabilityof EELS is realized when it is combined with the power of a TEM or STEM to form an Analytical Electron Microscope (AEM). This combination allows the analyst to perform spatially resolved nondestructive analysis with high-resolution imaging (e 3 A). Thus, not only can the analyst observe the microstructure of interest (see the TEM article) but, by virtue of the focusing ability of the incident beam in the electron microscope, he or she can simultaneously analyze a specific region of interest. Lateral spatial resolutions of regions as small as 10 A in diameter are achievablewith appropriate specimens and probe-forming optics in the electron microscope.

-

Basic Principles

EELS is a direct result of the Coulombic interaction of a fast nearly monochromatic electron beam with atoms in a solid. As the incident probe propagates through the 136


Chapter 3

Incident Electrons EJccted Inner Shell Electron

#e.-

a Inelastically Electron: Elastically Scaftered Electron: A E = 0

Figure l

Scattered AE > 0

b

M L

'\

(a) Excitation of inner shells by Coulombic interactions. (b) Energy level diagram illustrating excitation from inner shell and valence band into the conduction band and the creation of a corresponding vacancy.

specimen it experience elastic scatteringwith the atomic nuclei and inelastic scattering with the outer electron shells (Figure la). The inelastic scattering, either with the tightly bound inner shells or with the more loosely bound valence electrons, causes atomic electrons to be excited to higher energy states or, in some cases, to be ejected completely from the solid. This leaves behind a vacancy in the corresponding atomic level (Figure Ib). The complementary analysis techniques of X-ray and Auger spectroscopy (covered in other artides in this book) derive their signals from electron repopulation of the vacancies created by the initial excitation event. M e r the interaction, the energy distribution of the incident electrons is changed to reflect this energy transfer, the nature and manifestation of which depends upon the specific processes that have occurred. Because EELS is the primary interaction event, all the other analytical signals derived from electron excitation are the result of secondary decay processes. EELS, therefore, yields the highest amount of information per inelastic scattering event of all the electron column-based spectroscopies. Historically, EELS is one of the oldest spectroscopic techniques based ancillary to the transmission electron microscope. In the early 1940s the principle of atomic level excitation for light element detection capability was demonstrated by using EELS to measure CyN, and 0. Unfortunately, at that time the instruments were limited by detection capabilities (film) and extremely poor vacuum levels, which caused severe contamination of the specimens. Twenty-five years later the experimental technique was revived with the advent of modern instrumentation.' The basis for quantification and its development as an analytical tool followed in the mid 1970s. Recent reviews can be found in the works by Joy, Maher and Silcox;' C ~ l l i e xand ; ~ the excellent books by ðer4 and E g e r t ~ n . ~ 3.2

EELS

137

0

50

100

150

200

Energy Loss (eV)

Figure2

Example of an energy-loss spectrum, illustrating zero loss, and low-loss valence band excitations and the inner shell edge. The onset at 111 aV identifies the material as beryllium. A scale change of lOOX was introducedat 75 eV for display purposes.

Figure 2 is an experimental energy-lossspectrum measured h m a thin specimen of beryllium. At the I&, at zero energy loss, is a large, nearly symmetric peak which represents electrons that have passed through the specimen suffering either negligible or no energy losses. These are the elastically scattered and phonon-scattered incident electrons. Following this peak is the distribution of inelastically scattered electrons, which is generally broken up into two energy regimes for simplicity of discussion. The low-loss regime extends (by convention) from about 1eV to 50 eV, and exhibits a series of broad spectralfeatures related to inelastic scatteringwith the valence electron structure of the material. In metallic systems these peaks arise due to a collective excitation of the valence electrons, and are termed p h o n oscillations or peaks. For most materials these peaks lie in energy range 5-35 eV. Beyond this energy and extending fbr thousands of eV one observes a continuously decreasing background superimposed upon which are a series of “edges” resulting from electrons that have lost energy corresponding to the creation of vacancies in the deeper core levels ofthe atom (K, L3 L2,L,, M,, and so forth). The edges are generally referred to by the same nomenclature as used in X-ray absorption spectroscopy. The energy needed to ejected electrons amounts to the binding energy of the respective shell (Figure lb), which is characteristic for each element. By measuring the threshold energy of each edge the andyst can determinethe identity of the atom giving rise to the signal, while the net integrated intensity for the edge can be analyzed to obtain the number of atoms producing the signal. This is the basis of quantitative compositionalanalysis in EELS. 138


Chapter 3

Figure 3

Schematic representation of EELS analyzer mounted on a TEM/ STEM.

The energy regime most frequently studied by EELS is 0-3 keV. Higher energy losses can be measured; however, a combination of instrumental and specimenrelated limitations usually means that these higher loss measurements are more favorable for study by alternative analytical methods, such as X-ray energy-dispersive spectroscopy (see the article on EDS). The practical consequence of this upper energy limit is that for low-Zelements (1 I Z I 11) one studies K-shell excitation; for medium-2 materials (12 I Z I 45), L shells; and for high-Z solids (19 I Z I 79), M, N, and 0 shells (the latter for Z > 46). It is also important to realize that not all possible atomic levels are observed in EELS as edges. The transitions from initial states to final states generally must obey the quantum number selection rules: A j = 0, f l , and A1 = fl. Hence some atomic energy levels, although discrete and well defined, are not discernible by EELS. Hydrogen and helium are special cases that should be mentioned separately. These elements have absorption edges at 13 eV and 22 eV, respectively. These vdues lie in the middle of the low-loss regime, which is dominated by the valence band scattering. Thus, while the physics of inelastic scattering processes dictates that the edges will be present, u s d l y they will be buried in the background of the more intense valence signal. In special cases, for example, when the plasmon losses are well removed, or when the formation of hydrides6 occurs,presence of hydrogen and helium may be measured by EELS. The instrumentation used in EELS is generally straightforward. Most commercial apparatus amount to a uniform field magnetic sector spectrometer located at the end of the electron-optical column of the TEM or STEM (Figure 3). Electrons that have traversed the specimen are focused onto the entrance plane of the spectrometer using the microscope lenses. Here the electrons enter a region having a uniform magnetic field aligned perpendicular to their velocity vector, which causes them to be deflected into circular trajectories whose radii vary in proportion to their

-

3.2

EELS

139

velocity or energy and inversely with the magnetic field strength (R= [ n q p ] / e B ) . Location of a suitable detector system at the image plane of the spectrometer then allows the analyst to quantitatively measure the velocity-energy distribution. More complex spectrometers that use purely electrostatic or combined electrostatic and electromagnetic systems have been developed; however, these have been noncommercial research instruments and are not used generally for routine studies. More recently, elaborate imaging Spectrometers also have been designed by commercial firms and are becoming incorporated into the column of TEM instruments. These newer instruments show promise in future applications, particularly in the case of energy-loss filtered imaging. Low-Loss Spectroscopy

As we outlined earlier, the low-loss region of the energy-lossspectrum is dominated by the collective excitations of valence band electrons whose energy states lie a few tens of eV below the Fermi level. This area of the spectrum primarily provides information about the dielectric properties of the solid or measurements of valence electron densities. As a fast electron loses energy in transmission through the specimen its interaction-i.e., the intensity of the measured loss spectrum I(E)-can be related to the energy-loss probability P(E,q), which in turn can be expressed in terms of the energy-loss function Im[-&(E, q)] from dielectric the01-y.~Here q is the momentum vector, and E = ( ~ +1 i EZ) is the complex dielectric function of the solid.4 By applying a Garners-Kronig analysis to the energy-loss function (Im [-&-'(E, q)]), the real and imaginary parts ( ~ 1 ~, 2 of) the dielectric function can be determined. Using ~1 and ~ 2 one , can calculate the optical constants (the refractive index q, the absorption index K, and the reflectivity R) for the material being exa~nined.~-~ In addition to dielectric property determinations, one also can measure valence electron densities from the low-loss spectrum. Using the simple free electron model one can show that the bulk plasmon energy (E> is governed by the equation:

where e is the electron charge, rn is its mass, is the vacuum dielectric constant, h is Planck's constant, and q is the valence electron density. From this equation we see that as the valence electron density changes so does the energy of the plasmonloss peak. Although this can be applied to characterization, it is infrequently done today, as the variation in with composition is small7and calibration experiments must be performed using composition standards. A recent application is the use of plasmon losses to characterize hydrides in solids6 Figure 4 shows partial EELS spectra from Mg, Ti, Zr, and their hydrides. The shift in the plasmon-loss peaks

5

140


Chapter 3

I

T

L kITiH1 .87

zs

zrq.6

. . . 1 . " 7 . ' 1 ' . . ' 1 . . .

I

10203040 Energy

Figure 4

10203040 Energy

0 10203040 Energy

Experimental low-loss profiles for Mg (10.01, Ti (17.2). Zr(16.6). and their hydrides MgH2(14.21, TiH,,, (20.01, and ZrH,,6 (18.11. The values in parentheses represent the experimental plasmon-loss peak energies in eV.

shows that the addition of hydrogen acts to increase the net electron density in these materials. Inner Shell Spectroscopy

The most prominent spectral feature in EELS is the inner shell edge profile (Figure 2). Unlike EDS, where the characteristic signal profiles are nominally Gaussian-shaped peaks, in EELS the shape varies with the edge type (K, L, M yetc.), the eiectronic structure, and the chemical bonding. This is illustrated in Figure 5, which compares spectra obtain from a thin specimen of NiO using both windowless EDS and EELS. The difference in spectral profiles are derived from the fact that different mechanisms give rise to the two signals. In the case of X-ray emission, the energy of the emitted photon corresponds to the energy differences between the initial and final states when a higher energy level electron repopulates the inner shell level, filling the vacancy created by the incident probe (Figure 1b). These levels are well defined and discrete, corresponding to deep core losses. The information derived is therefore mainly representative of the atomic elements present, rather than of the nuances of the chemical bonding oi electronic structure. EDS is most frequently used in quantitative compositional measurements, and its poor energy resolution -100 eV is due to the solid state detectors used to measure the photons and not the intrinsic width of the X-ray lines (about a few eV). By contrast, in EELS the characteristic edge shapes are derived from the excitation of discrete inner shell levels into states above the Fermi level (Figure 1b) and reflect the empty density of states above EF for each atomic species. The overall 3.2

EELS

141

300

400

500

600

700

800

900

1000

Energy (eV)

Figure 5

Comparison of spectral profiles measured from a specimen of NiO using EDS and EELS. Shown are the oxygen K- and nickel L-shell signals. Note the difkrence in the spectral shape and peak positions, as well as the energy resolution of the two spectroscopies.

shape of an edge can be approximately described using atomic models, due to the fact that the basic wavefunctions of deep core electrons do not change significantly when atoms condense to form a solid. Thus, the different edge profiles can be sketched as shown in Figure 6. K-shell edges (s + p transitions) tend to have a simple hydrogenic-like shape. L-shell edges (p + s and p + d transitions) vary between somewhat rounded profiles (11 I Z I 17) to nearly hydrogenic-like, with intense “white lines” at the edge onset (19 I Z I 28, and again for 38 IZ I 46). In the fourth and fifth periods, these white lines are due to transitions from p to d states. M shells generally tend to be of the delayed-onset variety, due to the existence of an effective centrifugal barrier that is typical of elements with final states having large I quantum numbers. White lines near the M-shell edge onsets are observed when empty d states (38 I Z I 46) or f states (55 IZ I 70) occur, as in the case of the L shells. N and 0 shells are variable in shape and tend to appear as large, somewhat symmetrically shaped peaks rather than as “edges.”

L3L2

Figure6

142

L1

Ms M 4 M3 MzMi

N45..

023..

Schematic illustration of K, L, M, N and 0 edge shapes; the “white lines” sometimes detected on Land M shells are shown as shaded peaks at the edge onsets. In all sketches the backgroundshape has been omitted for clarity.


Chapter 3

K

Net Edge Intensity

I

Extrapolated Background

I

Energy Loss(eV)

450.

Figure 7

750

Details of oxygen K shell in NiO, illustrating NES and EXELFS oscillationsand the measurement of the integrated edge intensity used for quantitative concentration determination.

It is important to note that although specific edge profiles follovr these generic shapes somewhat, they can deviate significantly in finer details in the vicinity of edge onsets. This structure arises due to solid state effects, the details of which depend upon the specificstate (both electronicand chemical) of the material under scrutiny. Because of this strong variation in edge shape, experimental libraries of edge profiles also have been documenteds9 and have proven to be extremely useful supplementary tools. (Calculation of the detailed edge shape requires a significant computational effort and is not currently practical for on-line work.) These solid state effects also give rise to additional applications of EELS in materials research, namely: measurements of the d-band density of states in the transition metal systems,lo and chemical state determinations" using the near-edge structure. The former has been used successfullyby several research groups, while the latter application is, as yet, seldom used today in materials science investigations. A more detailed description of near edge structure requires that one abandon simple atomic models. Instead, one must consider the spectrum to be a measure of the empty locui dtnsity of states above the Femi level of the elemental species being studied, scaled by the probability that the particular transition will occur. A discussion ofsuch an undertaking is beyond the scope ofthis article, but EELS derives its capabilitiesfor electronicand chemical bonding determinations h m the near-edge structure. Calculation ofthis structure, which is due to the joint density of states, is involved and the studies of Grunes et ala" represent some of the most complete work done to date. The near-edge structure covers only the first k v tens of eV beyond the edge onset; however, as we can see intensity oscillations extend for hundreds of eV past the edge threshold. This extended energy-loss fine structure (EXELFS) is analogous to the extended absorption fine structure (EXAFS) visible in X-ray absorption spectroscopy. An example of these undulations can be seen in the weaker oscillations extending beyond the oxygen K edge ofFigure 7. The anal3.2

EELS

143

ysis of EXELFS oscillations can be taken virtually from the EXAFS literature and applied to EELS data, and allows the experimentalist to determine the nearest neighbor distances and coordination numbers about individual atomic species.l 3 Quantitative Concentration Measurements

The principles of quantitative concentration measurement in EELS is straightforward and simpler than in EDS. This is due to the fact that EELS is the primary interaction event, while all other electron-column analytical techniques are the result of secondary decay or emission processes. Thus, all other electron microscope-based analytical spectroscopies (EDS, Auger, etc.) must incorporate into their quantitative analysis procedures, corrections terms to account for the variety of competing processes (atomic number effects, X-ray fluorescenceyields, radiative partition hnctions, absorption, etc.) that determine the measured signal. In EELS, the net integrated intensity in the kth edge profile for an element corresponds simply to the number of electrons which have lost energy due to the excitation of that particular shell. This is related to the incident electron intensity (Io)multiplied by the cross section for ionization of the kth edges oKtimes the number of atoms in the analyzed volume (N):

IK

= NOKI0

Here IKis the net intensity above background over an integration window of A E (Figure 7),while Io is the integrated intensity of the zero-loss peak (Figure 2). Generally the background beneath an edge is measured before the edge onset and extrapolated underneath the edge using a simple relationship for the background shape: BG = AER.Here E is the energy loss, and A and R are fitting parameters determined experimentally from the pre-edge background. From Equation (2),one can express the absolute number of atoms/cm2 as:

Hence by measuring IK and Io and assuming OK is known or calculable, the analyst can determine N Using a hydrogenic model, Egerton5 has developed a set of FORTRAN subroutines (SigmaK and SigmaL) that are used by the vast majority of analysts for the calculation of K- and L-shell cross sections for the elements lithium through germanium. Leapman et al.14 have extended the cross section calculations. Using an atomic Hartree-Slater program they have calculated K-, L-, M-, and some N-shell cross sections, however, these calculations are not amenable to use on an entry-level computer and require substantial computational eff01-t.'~They do, however, extend the method beyond the limits of Egerton's hydrogenic model. Tabular compilations of the cross section are generally not available, nor do they 144


Chapter 3

tend to be useful, as parameters used in calculations seldom match the wide range of experimental conditions employed during TEM- or STEM-based analysis. An alternative approach to the quantitative analysis formalism is the ratio method. Here we consider the ratio of the intensities of any two edges A and B. Using Equation (3) we can show that

The elegance of this relationship rests in the fact that all the information one needs to measure the relative concentration ratio of any two elements is simply the ratio of their integrated edge profiles, Io having canceled out of the relationship. This ratio method is generally the most widely used technique for quantitative concentration measurements in EELS. Unfortunately, the assumptions used in deriving this simple relationship are never l l l y realized. These assumptions are simply that electrons scattering from the specimen are measured over allangles and for all energy losses. This is physically impossible, since finite angular and energy windows are established or measured in the spectrum. For example, referring to Figure 7, we see that in NiO the Ni L-shell edge is superimposed upon the tail of the oxygen K-shell edge and clearly restricts the integration energy window for oxygen to about 300 eV. Similarly it is impossible in a TEM or STEM to collect all scattered electrons over 'II: s R an upper limit of about 100 mR is practically attainable. A solution to this problem was devised by Egerton5 and can be incorporated into Equations (3) and (4) by replacing IAby IA(AE, p) and OA by OA(AE, p), since we measure over a finite energy (AE) and angular window (p). The quantity oA(AE, p) is now the partial ionization cross section for the energy and angular windows of AEand p, respectively. Using this ratio approach to quantification, accuracies of +5-10% at. for the same type edges (i.e., both K or L) have been achieved routinely using Egerton's hydrogenic models. When dissimilar edges are analyzed (for example one K and one L shell), the errors increase to fl5-20% at. The major errors here result from the use of the hydrogenic model to approximate all edge shapes. Although these errors may sound relatively large in terms of accuracy for quantification, it is the simplicity of the hydrogenic model that ultimately gives rise to the problem, and not the principle of EELS quantification. Should it be necessary to achieve greater accuracy, concentration standards can be developed and measured to improve accuracy. In this case, standards are used to accurately determine the experimental ratio (og(AE, P)/oA(AE p) by measuring IA/IB and knowing the composition N A / N B . These oB/oA d u e s are used when analyzing the unknown specimen, and accuracies to 1% at. can be obtained in ideal cases. When employing standards, it is essential that the near-edge structure does not vary significantly between the unknown and the standard, since in many cases near-edge structure 3.2

EELS

145

I

I

"

'

"

'

a00

'

~

"

'

'

300

Enerpy

Figure 8

"

I

'

400

'

~

'

I

~

.

~

'

500

(eV)

Illustration of the decrease in the edge/ background ratio for the 6, (-188 eV) and NK(-399 eV) shell! in EELS. In the data sets,the upper profile is from the Ficker region (-2000 A) of the BN specimen while the lower is thinner (-200 A). Note the logarithmicvertical scale.

contributes substantially to the net integrated edge profiles. This, unfortunately, is usually a difficult situation to realize. As a practical note, standards ofien are not used due to the fact that they require the analyst to prepare accurate multielement standards in TEM form for each elemental system to be studied and for every set of operating conditions used during the analysis of the unknown. Limitationsand Specimen Requirements

The single most important limitation to the successful application of EELS to problems in materials characterization relates to the specimen, namely, its thickness. Being a transmission technique it is essential that the incident beam penetrate the specimen, interact, and then enter the spectrometerfor detection. As h e specimen thickness increases, the likelihood of inelastic scattering increases, and hence the EELS signal increases. Unfortunately, the background signal increases at a faster rate than that of the characteristic edges. This results in the edges becoming effectively lost in the background, as illustrated in Figure 8, which shows the decrease in the edge-to-background ratio obtained from different thicknesses of a specimen of boron nitride. As a general rule, if h is the mean free path for inelastic scattering, the specimen thickness tshould not exceed values of t/ h = 1 and preferably should be < 0.5 to minimize the adverse effects of multiple scattering. The mean free path h is a function of the atomic number and the accelerating voltage. At 100 kV,h is about 1200 A for aluminium, decreasing to -900 A at nickel, and reaching 4 0 0 A for gold. Increasing the acceleratingvoltage of the electron microscope reduces multiple inelastic scattering somewhat; for example, increasing the incident beam voltage from 100 to 300 kV increases h by a factor of -1.8, and going from 100 to 1000 kV yields a factor of -2.5. However, increasing the voltage introduces another set of problems for the experimentalist, that is, electron irradiation (displacement) damage. In this situation, the high-energy electrons have SUR14.6


Chapter 3

cient energy to displace atoms from their normal lattice sites and, in some cases, literally to sputter holes through the specimen.

Conclusions The combination of EELS with a TEM or STEM yields a powerful tool for the microcharacterization of materials. Its primary applications are in ultrahigh spatial resolution spectroscopy of thin electron-transparentsolids. With optimized specimens, EELS can be used to obtain the local elemental composition of a specific region of interest on the specimen, and with more detailed calculations can provide information concerning the electronic or chemical states of the sample. EELS can be applied to any specimen that can be prepared for observation in the TEM or STEM. Future developments will concentrate in the areas of higher speed data acquisition using one- and two-dimensional parallel detectors for combined spectroscopy and parallel imaging, ultrahigh-energy resolution spectroscopy in the 50100 meV range, and advanced software to make routine the more complex data analyses. Related Altieles in the fncydopedia

TEM, STEM, EDS, EXAFS, NEXAFS, X P S , and UPS, REELS

References i

A. V Crewe, M. Isaacson, and D. E. Johnson. Rev. Sci. Ins& 42,411,

1971. 2

3

4

5

6

3.2

Introduction to Am&icul Electron Microscopy. (J.J. Hren, D. C. Joy, and J. I. Goldstein, eds.) Plenum Press, 1979.A good overview of analyticalelectron microscopy. C. Colliex. In: Advances in Opticaland E h m n Mimscopy (RBarer and YE. Cosslett, eds.) Academic Press, 1984,Volume 9.This chapter contains a concise, but detailed, treatment of EELS with significant references to the major studies done. H. Raether. Springer Tmcts in Modern Physics. 88,1980.This book details the wealth of information contained in the low-loss spectrum, and treats the mathematics in considerabledetail. R. F. Egerton. Ehctron Energy Loss Spectrometry in the Ehctron Microscope. Plenum Press, 1986. This is a comprehensivetext on the use of EELS in the TEM. It covers instrumentation, theory and practical applications. N. J. Zaluzec, T. Schober, and B. W. Veal. In: Atza&icaZ Electron Mimsc o p p I 9 8 2 Proceedings of the Worksbop at k i l Colorado. San Francisco Press,p. 191. EELS

147

D. B. Williams and J. W. Edington. J Micmsc. 108,113, 1976. 8 N. J. Zaluzec. Uhamicrosco~. 9,319,1982; andj. &Physique. C245 (2), 1984.This work is also availablegratis from the author, at EM Center, Argonne National Laboratory Materials Science Division-212, Argonne, IL 60439, USA. 9 C. C. Ahn and 0. L. Krivanek An Atlas ofElecmn Energy Loss Spectra. Available from Gatan Inc., Pleasanton, CA 94566, USA. io T. I. Morrison, M. B. Brodslo/, and N. J. Zaluzec. Pbys. Rev.B. 32, (5) 3107,1985. 11 M. Isaacson. In: Microbeam Analysis in Biology. (C. I? Lechene and R. R Warner, eds.) Academic Press, New York, 1979, p.53. 12 L. A. Grunes, R. D. Leapman, C. N. Wilker, R Hoffmann, a n d k B. Kunz. Pbys.Rev. B. 25,7157, 1982. 13 M. M. Disko, 0. L. Krivanek, and I? Rez. Pbys.Rev. B. 25,4252,1982. 14 R. D. Leapman, I? Rez, and D. E Mayers. j. Cbem. Pbys. 72,1232,1980. 7

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Chapter 3

CL

3.3

Cathodoluminescence B . G . YACOBI

Contents

Introduction Basic Principles Instrumentation Quantification General Applications and Examples Artifacts Conclusions

Introduction

Cathodoluminescence (CL), i.e., the emission of light as the result of electronbeam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequently this phenomenon led to the discovery of the electron. Currently, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerful tools for the microcharacterization of the electronic properties of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques indude: 1 Uniformity characterization of luminescent materials (e.g., mapping of defects and measurement of their densities, and impurity segregation studies) 3.3

CL

149

2

Obtaining information on a material’s electronic band structure (related to the fundamental band gap) and analysis of luminescence centers

3

Measurements of the dopant concentration and of the minority carrier diffusion length and lifetime

4

Microcharacterization of semiconductor devices (e.g., degradation of optoelectronic devices)

5

Analysis of stress distributions in epitaxial layers

6

In-situ characterization of dislocation motion in semiconductors

7

Depth-resolved studies of defects in ion-implanted samples and of interface states in heterojunctions.

In CL microscopy, luminescence images or maps of regions of interest are displayed, whereas in CL spectroscopy a luminescence spectrum from a selected region of the sample is obtained. The latter is analogous to a point analysis in X-ray microanalysis (see the article on EPMA). However, unlike X-ray emission, cathodoluminescence does not identify the presence of specific atoms. The lines of characteristic X-rays, which are emitted due to electronic transitions between sharp innercore levels (see the articles on EDS, EPMA, or XRF), are narrow and are largely unaffected by the environment of the atom in the lattice. In contrast, the CL signal is generated by detecting photons (in the ultraviolet, visible, and near-infrared regions of the spectrum) that are emitted as the result of electronic transitions between the conduction band, or levels due to impurities and defects lying in the fundamental band gap, and the valence band. These transition energies and intensities are affected by a variety of defects, by the surface of the material, and by external perturbations, such as temperature, stress, and electric field. Thus, no universal law can be applied in order to interpret and to quantify lines in the CL spectrum. Despite this limitation, the continuing development of CL is motivated by its attractive features: 1

2

3

4

CL is the only contactless method (in an electron probe instrument) that provides microcharacterization of electronic properties of luminescent materials. A CL system attached to a scanning electron microscope (SEM) provides a powerful means for the uniformity studies of luminescent materials with the spatial resolution of less than 1 pm. The detection limit of impurity concentrations can be as low as lo1*atoms/cm3, which is several orders of magnitude better than that of the X-ray microanalysis mode in the SEM. CL is a powerful tool for the characterization of optical properties of wide bandgap materials, such as diamond, for which optical excitation sources are not readily available.

150


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Since the excitation depth can be selected by varying the electron-beam energy, depth-resolved information can be obtained. 6 In optoelectronic materials and devices, it is the luminescence properties that are of practical importance. CL studies are performed on most luminescent materials, including semiconductors, minerals, phosphors, ceramics, and biological-medical materials. 5

Basic Principles The Excitation Process

As the result of the interaction between keV electrons and the solid, the incident electron undergoes a successive series of elastic and inelastic scattering events, with the range of the electron penetration being a function of the electron-beam energy: R, = ( M p ) where Eb is the electron-beam energy, k and cc depend on the atomic number of the material and on &, and p is the density of the material. Thus, one can estimate the so-called generation (or excitation) volume in the material. The generation factor, i.e., the number of electron-hole pairs generated per incident beam electron, is given by G = Eb (l-y)/Ei, where 4 is the ionization energy (i.e., the energy required for the formation of an electron-hole pair), and y represents the fractional electron-beam energy loss due to the backscattered electrons.

c,

Luminescence Processes

The emission of light in luminescence processes is due to an electronic transition (relaxation) between a higher energy state, Ez, and an empty lower energy state, El. (The state 4 is the excited state caused by the electron-beam excitation process.) The energy, or the wavelength of the emitted photon, bv = b c A = 4 -El. The wavelength h (in pm) of a photon is related to the photon energy E (in ev) by 3c E 1.2398/E In wide band-gap materials luminescence occurs in the visible range (from about 0.4 to 0.7 pm, corresponding to about 3.1 to 1.8 ev). In many cases, luminescence also occurs at longer wavelengths in the near-infrared region. For a simplified case, one can obtain' the rate of CL emission, LCL = fq Glb/c, where f i s a hnction containing correction parameters of the CL detection system and that rakes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses;' q is the radiative recombination efficiency (or internal quantum efficiency); l b is the electronbeam current; and e is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distinguish between radiative and nonradiative processes in a quantitative manner. One should also note that depends on various factors, such as temperature, the presence of defects, and the 3.3

CL

151

Figure 1

Schematicdiagram of luminescence transitionsbetween the conductionband (Ec),the valence band (4). and exciton (EE),donor (Eo.) and acceptor (EA)levels in the luminescent material.

particular dopants and their concentrations. One result of the analysis of the dependence of the CL intensity on the electron-beam energy indicates the existence at the surface of a dead layer, where radiative recombination is absent.2 In inorganic solids, luminescence spectra can be categorized as intrinsicor extrinsic. Intrinsic luminescence, which appears at elevated temperatures as a near Gaussian-shaped band of energies with its peak at a photon energy hv, z Eg, is due to recombination of electrons and holes across the fundamental energy gap Eg (see Figure 1). Extrinsic luminescence, on the other hand, depends on the presence of impurities and defects. In the analysis of optical properties of inorganic solids it is also important to distinguish between direct-gap materials (e.g., GaAs and ZnS) and indirect-gup materials (e.g., Si and Gal?).This distinction is based on whether the valence band and conduction band extrema occur at the same value of the wave vector k i n the energy band E(k) diagram of the particular solid. In the former case, no phonon participation is required during the direct electronic transitions. (A phonon is a quantum of lattice vibrations.) In the latter case, phonon participation is required to conserve momentum during the indirect electronic transitions; since this requires an extra particle, the probability of such a process occurring is significantly lower than that of direct transitions. Thus, fundamental emission in indirect-gap materials is relatively weak compared with that due to impurities or defects. A simplified schematic diagram of transitions that lead to luminescence in materials containing impurities is shown in Figure 1. In process 1 an electron that has been excited well above the conduction band edge dribbles down, reaching thermal equilibrium with the lattice. This may result in phonon-assisted photon emission or, more likely, the emission of phonons only. Process 2 produces intrinsic luminescence due to direct recombination between an electron in the conduction band 152


Chapter 3

and a hole in the valence band, and this results in the emission of a photon of energy hv E Eg'Process 3 is the exciton (a bound electron-hole pair) decay observable at low temperatures; free excitons and excitons bound to an impurity may undergo such transitions. In processes 4,5,and 6, transitions that start or finish on localized states of impurities (e.g., donors and acceptors) in the gap produce extrinsic luminescence, and these account for most of the processes in many luminescent materials. Shallow donor or acceptor levels can be very close to the conduction and valence bands; to distinguish between the intrinsic band-to-band transitions and those associated with shallow impurity transitions, measurements have to be performed at cryogenic temperatures, where CL spectra are sharpened into lines corresponding to transitions between well-defined energy levels. Process 7 represents the excitation and radiative deexcitation of an impurity with incomplete inner shells, such as a rare earth ion or a transition metal. It should be emphasized that lattice defects, such as dislocations, vacancies, and their complexes with impurity atoms, may also introduce localized levels in the band gap, and their presence may lead to the changes in the recombination rates and mechanisms of excess carriers in luminescence processes. Spatial Resolution

The spatial resolution of the CLSEM mode depends mainly on the electron-probe size, the size of the excitation volume, which is related to the electron-beam penetration range in the material (see the articles on SEM and EPMA), and the minority carrier diffusion. The spatial resolution also may be affected by the signal-to-noise ratio, mechanical vibrations, and electromagneticinterference. In practice, the spatial resolution is determined basically by the size of the excitation volume, and will be between about 0.1 and 1 pm' Instrumentation

Two general categoriesof CL analysis systems are wavelength nondispersive-versusdispersive, and ambient-versus-cryogenic temperature designs. The first categor). essentially leads to two basic CL analysis methods, microscopy and spectroscopy. In the former case, an electron microscope (SEM or STEM) is equipped with various CL detecting attachments, and thus CL images or maps of regions of interest can be displayed on the CRT. In the latter case an energy-resolved spectrum corresponding to a selected area of the sample can be obtained. CL detector designs differ in the combination of components used3. Although most of these are designed as SEM attach~nents,~ several CL collection systems were developed in dedicated STEMS.~ The collection efficiencies of the CL detector systems vary from several percent for photomultipliers equipped with light guides, to dose to 90% for systems incorporating ellipsoidal or parabolic mirrors coupled directly to a monochromator. A relatively simple and inexpensive, but powerful, CL detector using an 3.3

CL

153

optical fiber light collection system also has been devel~ped.~ In these designs, the signal from the photomultiplier can be used to produce micrographs and spectra. When the grating of the monochromator is bypassed, photons of all wavelengths falling on the photomultiplier produce the panchromatic (integral) CL signal. In the dispersive mode, for a constant monochromator setting and a scanning electron-beam condition, monochromatic micrographs can be obtained; and when the monochromator is stepped through the wavelength range of interest and the electron beam is stationary or scans a small area, CL spectra can be derived. The proper choice of a detector is important in CL measurements. In the visible range, photomultipliers are the most efficient detectors. For luminescence in the infrared range, solid state detectors, as well as Fourier transform spectrometry (FTS) can be used. For detailed quantitative analysis, the calibration of the CL detection system for its spectral response characteristics is important in most cases. Although in many applications noncryogenic CL system designs may be s u s cient, for detailed quantitative studies of impurities and defects in various materials it is necessary to use high-efficiency light-collection dispersive systems having the capability of sample cooling, preferably to liquid-helium temperatures. The advantages of sample cooling &e to increase the CL intensity, to sharpen the CL spectrum into lines corresponding to transitions between well-defined energy levels that allow the more reliable interpretation of CL spectra, and to reduce the rate of electron bombardment damage in electron-beam sensitive materials. Another basic approach of CL analysis methods is that of the CL spectroscopy system (having no electron-beam scanning capability),which essentially consists of a high-vacuum chamber with optical ports and a port for an electron gun. Such a system is a relatively simple but powerful tool for the analysis of ion implantationinduced damage, depth distribution of defects, and interfaces in semiconductors.' Optical CL microscopes are instruments that couple electron gun attachments to optical microscopes. Although such systems have a limited spatial resolution, they are used widely in the analysis of minerals.'

Quantification

As mentioned above, the interpretation of CL cannot be unified under a simple law, and one of the fundamental difficulties involved in luminescence analysis is the lack of information on the competing nonradiative processes present in the material. In addition, the influence of defects, the surfice, and various external perturbations (such as temperature, electric field, and stress) have to be taken into account in quantitative CL analysis. All these make the quantification of CL intensities difficult. Correlations between dopant concentrations and such band-shape parameters as the peak energy and the half-width of the CL emission currently are more reliable as means for the quantitative analysis of the carrier concentration. 154


Chapter 3

-

a

b

10 pm

Figure 2

H

10 urn

CL micrographs of Te-doped GaAs: dark-dot dislocation contrast (a) in GaAs doped with a Te concentration of I O l 7 ~ r n - ~ and ; dot-and-halo dislocation contrast (b) in GaAs doped with a Te concentration of 10l8~ 1 1 1 ~ ~ .

Nonradiative surface recombination is a loss mechanism of great importance for some materials (e.g., GaAs). This effect, however, can be minimized by increasing the electron-beam energy in order to produce a greater electron penetration range. A method for quantification of the CL, the so-called MAS corrections, in analogy with the ZAF correction method for X rays (see the article on EPMA), has been proposed' to account for the effects of the excess carrier concentration, absorption and surface recombination. In addition, a total internal reflection correction should also be included in the analysis, which leads to the MARS set of corrections. This method can be used for further quantification efforts that also should involve Monte Carlo calculations of the generation of excess carriers. General Applications and Examples

Major applications of CL microscopy and spectroscopy in the analysis of solids have been listed in the Introduction. Some specific examples of CL applications are outlined below. An example of the uniformity characterization, as well as of the analysis of the electrically active defects, is shown in Figure 2. These CL micrographs demonstrate two different forms of dislocation contrast (dark-dot and dot-and-halo contrast) for GaAs crystals doped with Te concentrations of 10'' cm-3 (Figure 2a) and 10" cm-3 (Figure 2b). The latter shows variations in the doping concentration around dislocations. This figure also demonstrates that CL microscopy is a valuable tool for determining dislocation distributions and densities in luminescent materi3.3

CL

155

Figure3

Monochromatic CL image (recorded at 1.631 eV) of quantum well boxes, which appear as bright spots?

als. Reliable measurements of dislocation densities up to about IO6 cm-2 can be made with the CL image. An example of the CL microcharacterization of an array of GaAs/AlGaAs quantum well (QW>boxes’ is presented in Figure 3, which shows the CL monochromatic image recorded at the energy corresponding to one of the characteristic luminescence lines (i.e., 1.631 eV). In such structures, the carriers are confined by surrounding a smaller band-gap semiconductor layer with wider band-gap layers. Confinement of carrier motion to 0 degrees of freedom will be obtained for the smaller band-gap layer in the form of a box.’ The monochromatic CL image shows nonuniformities in the luminescence intensity from one box to another, since not all the QW boxes are identical due to variations in the confining potential between them that result from the presence of residual processing-induced damage.’ Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures,10such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties; it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal, 156


Chapter 3

4

a

= 818 nm

b

X=824nm

c

-

X=832nm

100 pm

Figure 4

MonochromaticCL images of the GaAs /Si sample recordedat 818 nm (a), 824 nm (b), and 832 nm (c). Microcracksare indicated by arrows in (a). The sample temperature is about 20 K."

for example, variations in stress associatedwith the patterning of GaAs layers grown on mismatched substrates." An example describing stress variations and relief due to patterning in GaAs grown on Si substrates is shown in Figure 4, which presents monochromatic CL images of a GaAs layer at 8 18,824, and 832 nm. These images demonstrate that the convex corners and the edges in the patterned regions emit at shorter wavelengths compared to the interiors of these regions. Detailed analysis of the CL spectra in different regions of a GaAs layer indicates strong variations in stress associated with patterning of such layers.'' An example of CL depth-resolved analysis of subsurface metal-semiconductor interfaces, using an ultrahigh-vacuum CL system,' is shown in Figure 5. This figure presents CL spectra of ultrahigh vacuum-cleaved CdS before and after 50-A Cu deposition and pulsed laser annealing.' The deposition of Cu produces a weak peak at about 1.27 eV, in addition to the CdS band-edge emission at 2.42 eV. Pulsed laser annealing with an energy density of 0.1 J/cm2 increases the intensity of this peak, which is related to Cu2S compound formation.' This specific example clearly indicates that low-energy CL spectroscopycan be used effectively in the analysis of chemical interactions at buried metal-semiconductor interfaces. As mentioned earlier, CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. In addition, electron-beam excitation of solids may produce much greater carrier generation rates than typical optical excitation. In such cases, CL microscopy and spectroscopy are valuable methods in identifying various impurities, defects, and their complexes, and in providing a powerful means for the analysis of their distribution, with spatial resolution on the order of 1 pm and less. l 1 3.3

CL

157

n

2kV

CdS+ 5OA Cu Laser Annealed

5 0 A Cu

Cleaved

\ L

to

1.4

Figure 5

1.0 2.2 2.6 EnergyleVl

I

3.0

CL spectramofuttrahigh vacuum-cleaved WS before and after in situ deposition of 50 A of Cu, and after in situ laser annealing using an energy density of 0.1 J /cm2. The electron-beam voltage is 2 kV!

Artifacts

Artifacts in CL analysis of materials may arise from luminescent contaminants, from scintillations caused by backscattered electrons striking components of the CL system, or from incandescence from the electron gun filament reaching the detector. Most of these may be eliminated by chopping the electron beam and detecting the CL signal with a lock-in amplifier. In the CL spectra, ghost peaks may appear due to total internal reflection and self-absorption.8 Prolonged electronbeam irradiation may cause changes in the CL intensity, localized heating at high beam currents, or may lead to surface contamination of the specimen. To alleviate the latter problem, it is recommended to replace the diffusion pump by a turbomolecular pump, or to use liquid-nitrogen cooled shields around the specimen. Spurious signals also may arise due to charging in nonconducting samples. As a general rule, the electron-beam damage is minimized by maximizing the beam voltage, minimizing the beam current, and using minimum detection times. It should be noted that during CL observations intensity variations may arise due to sample morphology (e.g., surface roughness), which may lead to nonuniform excitation and to local variations in optical absorption and reflection losses. 158


Chapter 3

Conclusions

In summary, CL can provide contactless and nondestructive analysis of a wide range of electronic properties of a variety of luminescent materials. Spatial resolution of less than 1 pm in the CGSEM mode and detection limits of impuriry concentrations down to lo'* at/cm3 can be attained. CL depth profiling can be performed by varying the range of electron penetration that depends on the electron-beam energy; the excitation depth can be varied fiom about 10 nm to several w for electron-beam energies ranging between about 1 keV and 40 keV. The development of quantitative CL analysis is the most challenging issue. With hrther development of interpretive theory and with the trend toward the computerization of electron microscopy, quantitative CL analysis should become feasible. Extensions in wavelength, into both the infiared and the ultraviolet ranges will continue, motivated by increasing interest in narrow band-gap semiconductors and wide band-gap materials. Applications of CL to the analysis of electron beam-sensitive materials and to depth-resolved analysis of metal-semiconductor intehces' by using low electronbeam energies (on the order of 1 kev) will be extended to other materials and structures. The continuing development of CL detection systems, cryogenic stages, and signal processing and image analysis methods will further motivate studies of a wide range of luminescent materials, including biological specimens.l2


EPMA, SEM, STEM, TEM, and PL References

2

B. G. Yacobi and D. B. Holt. CathodoLuminescenceMicroscopy oflnorganic SoLiA. Plenum, 1990. D. B. Wittry and D. E Kyser. J Appl. Phys. 38,375, 1967.

3

D. B. Hoit. In: Microscopy ofSemiconductingMaterials.IOP, Bristol, 1981,

1

4

5 6

7

8

3.3

p.165. l? M. PetrofK D. V.Lang, J. L. Strudel, and R. A. Logan. In: Scanning Ehctron Microscopj SEM Inc., Chicago, 1978, p. 325. M. E. Hoenk and K. J. Vahala. h. Sci. Imtr 60,226,1989. L. J. Brillson and R. E. Viturro. ScanningMicroscoB 2,789, 1988. C. E. Barker and T. Wood. In: Process Mineralogy 1/1. The Metallurgical Society ofAIME, 1987, p. 159. c.A. warwick. Scanning Microscopy 1,5 1, 1987. CL

159

9

IO

11

12

I? M. PetroK In: Microscopy of Semiconducting Materiah. IOP, Bristol, 1987, p. 187. B. G. Yacobi, S. Zemon, C . Jagannath,and I? Sheldon. J. Cryst. Growth 95,240, 1989. A. T. Collins and S. C. Lawson. J. Pbys. Cond. Matter 1,6929, 1989; L. H. Robins, L. I? Cook, E. N. Farabaugh, and A. Feldman. Pbys. Rea B39,13367,1989. D. B. Holt. In: Am& of Organic and Biological Sufaces. (I? Echlin, ed.) Wiley, New York, 1984, p.30 1.

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3.4

STEM Scanning Transmission Electron Microscopy C H A R L E S E. L Y M A N

Contents

9

Introduction Basic Principles Common Modes of Analysis and Examples Sample Requirements Artifacts Conclusions

Introduction

The Scanning Transmission Electron Microscope (STEM) produces images of the internal microstructure of thin specimens using a high-energy scanning electron beam, in the same way a Scanning Electron Microscope (SEM) produces images of bulk surfaces. The term STEM is also used to describe the group of crystallographic and compositional analysis methods known collectively as Analytical Electron Microscopy (AEM):Convergent Beam Electron Diffraction (CBED), X-ray microanalysis by Energy-Dispersive Spectrometry (EDS), and Electron EnergyLoss Spectrometry (EELS).l" Many STEM images are similar to images from the Transmission Electron Microscope (TEM), and in certain modes the STEM is capable of resolving the atomic lattice of a solid and even single atoms on a thin support. The STEM is unrivaled in its abiliry to obtain high-resolution imaging combined with microanalysis from specimens that can be fashioned from almost any solid. Major applications include the analysis of metals, ceramics, electronic devices 3.4

STEM

161

Secondary

Diffracted and scattered electrons

Electron

Characteristic

, Energy loss electrons Zero loss and undeviated electrons

Figure 1

Signals generated when the focussed electron beam interacts with a thin specimen in a scanning transmission electron microscope (STEM).

and packaging, joining methods, coatings, composite materials, catalysts, minerals, and biological tissues. There are three types of instruments that provide STEM imaging and analysis to various degrees: the TEM /STEM, in which a TEM instrument is modified to operate in STEM mode; the SEM/STEM, which is a SEM instrument with STEM imaging capabilities; and dedicated STEM instruments that are built expressly for STEM operation. The STEM modes of TEM/STEM and SEM/STEM instruments provide useful information to supplement the main TEM and SEM modes, but only the dedicated STEM with a field emission electron source can provide the highest resolution and elemental sensitivity. Analysis capabilities in the STEM vary with the technique used. Crystallographic information may be obtained, including lattice parameters, Bravais lattice types, point groups, and space groups (in some cases), from crystal volumes on the order of m3 using CBED. Elemental identification and quantitative microanalysis have been developed for EDS and EELS. Detection limits for each technique are on the order of 0.1 wt % for one element combined with another. The EELS spectrum contains a rich variety of information concerning chemical bonding and dielectric constants in addition to elemental information. Since the STEM provides a through-section analysis (see Figure I), it is complementary to surface techniques and should be used in conjunction with them. Also analytical signals may be collected as the small STEM electron probe scans across the specimen, providing compositional images in addition to the images typical of the SEM and the TEM. Compositional images showing elemental distributions have been obtained with spatial resolutions in the range 5-50 nm. 162


Chapter 3

ELECTRON BEAM

SCANM IN A RASTER ON SPECINEN hision

berm

-

Figure 2

Schematic of a STEM instrument showing the principal signal detectors. The electron gun and lenses at the bottom of the figure are not shown.

The development of the STEM is relatively recent compared to the TEM and the SEM. Attempts were made to build a STEM instrument within 15 years after the invention of the electron microscope in 1932. However the modern STEM, which had to await the development of modern electronics and vacuum techniques, was developed by Albert Crewe and his coworkers at the University of Chicago.’ Basic Principles Electron Probe Formation

An electron gun produces and accelerates the electron beam, which is reduced in diameter (demagnified) by one or more ekctromagnetic electron lenses. Electromagnetic scanning coils move this small electron probe (i.e., the beam) across the specimen in a raster. Electron detectors beyond the specimen collect a signal that is used to modulate the intensity on a cathode-ray tube that is scanned in synchronism with the beam on the specimen. A schematic of the essential components in a dedicated STEM system is shown in Figure 2. The most important criterion for a STEM instrument is the amount of current in the small electron probe. Generally, 1 nA of probe current is required for high3.4

STEM

163

a Figure 3

b

Bright-field (a) and dark-field (b) STEM images of crushed ceramic particles dispersed on a "holey" carbon film supported on an electron microscope grid (shown at the right).

quality microanalysis. For TEM/STEM and SEM/STEM systems using thermionic electron sources (tungsten wire or LaBG), electron probes having diameters of 10-30 nm (measured as full width at half maximum) carry about 1 nA, and may be used for imaging and analysis. Smaller probes may be used for imaging, but the current may not be adequate for microanalysis. For the highest spatial resolution and analytical sensitivity, a STEM instrument with a field-emission electron gun must be used to provide 1 nA of current in an electron probe about 1-2 nm in diameter. These systems must use ultrahigh-vacuum technology at least in the electron gun, and preferably throughout the microscope. Imaging

The scanning electron beam produces a diffraction pattern beyond the specimen similar to that formed in the TEM, and in most cases the image may be interpreted as mass-thickness contrast, diffraction contrast, or phase contrast. Bright-field images are formed by using an aperture to select only the undeviated transmitted beam from the diffraction pattern Figure 2, as in TEM (see the article on TEM). In STEM, the electron signal is collected with either a scintillator-photomultiplier or a semiconductor detector. Bright areas in the image indicate regions of the specimen that suffered little or no interaction with the electron beam (see Figure 3a). Dark-field images may be obtained in two ways: by selecting a single diffracted beam ghkl to be collected on the detector; or by collecting all of the electrons diffracted or scattered beyond a certain minimum angle on an annular dark-field (ADF) electron detector. The former method gives images similar to the TEM dark-field images used for defect analysis in crystals (see the TEM article). The latter method provides a high-resolution, high-contrast image that is sensitive to specimen thickness and atomic number variations. Bright areas in the dark-field image 164


Chapter 3

indicate regions of the specimen that are thick, strongly diffracting, or of high atomic number (see Figure 3b). The detailed contrast in a STEM image, compared to a TEM image of the same specimen feature, depends on the incident electron beam convergence angle and the electron collection angle at the detector. The theorem of reciprocity states that if appropriate beam angles in TEM and STEM are made equivalent and the sample is inverted, then the STEM and TEM images of a thin specimen will be identical (see Cowley’). For example, if the STEM collection angle is reduced to a value typical of the TEM illumination angle, similar phase contrast lattice plane images and structure images may be observed in both STEM and TEM. Often, the STEM collection angle must be enlarged to provide an adequate signal level, which may alter the image contrast. Because of the scanning nature of image generation many other signals, such as secondary electrons and cathodoluminescence (light), also may be used for imaging. Convergent Beam Electron Diffraction

When the electron beam impinging on the specimen has a high convergence angle (i.e., is in the form of a cone as shown in Figure l), the electron diffraction pattern becomes an array of disks rather than an array of sharp spots as in TEM. The various manifestations of this type of electron diffraction pattern are known as Convergent Beam Electron Diffraction (CBED). The distance of each diffraction disk from the central beam may be calibrated to yield the interplanar d-value for a particular set of hklplanes. A diffraction disk containing no contrast detail is produced when a very thin region of a specimen ( e 0.1 pm) is under the beam. Other than providing high spatial resolution (on the order of the electron beam size) this pattern of blank disks contains no more information than the typical selected area electron diffraction pattern (see the article on TEM). Diffraction from thick crystals (0.1-0.5 pm) exhibits intensity variations within the disks caused by dynamicd diffraction effects (see Steeds’). In this case the symmetry of the intensity variations provides information about the symmetry of the crystal that can be used as a “fingerprint” for phase identification.’ Because the convergent electron beam senses the three-dimensional aspects of the specimen, CBED patterns from a crystal thicker than about 0.1 pm may be used to determine its point group, and often its space group.’ If the magnification of the diffraction pattern is made very small and the convergence angle made very large so that the disks overlap, rings of intensity called higher order Laue zone ( H O U ) ring may be observed. These rings indicate that diffraction has occurred from other layers of the reciprocal lattice. If the crystal is tilted so that the beam is parallel to the [loo], [OlO], and [OOl]crystal directions, the crystal lattice parameters along those directions may be determined. Also, fine details of a CBED pattern may yield a relative lattice parameter determination to better than 0.001 nm.

3.4

STEM

165

X-Ray Microanalysis

When energetic electrons bombard a solid, characteristicX rays from each element are generated that form the signal used in microanalysis. Characteristic X rays arise from de-excitation of atoms suffering inner-shell electron ionizations, and these X rays allow qualitative elemental identification and quantitative elemental composition determination. The X-ray signal is detected with an EDS placed close to the specimen inside the objective lens of the microscope (see the article on EDS). For materials science specimens, a quantification scheme for specific element pairs is well developed.'" The ratio of elemental concentrations of two elements in the thin specimen may be determined by multiplying the X-ray intensity ratios of these elements by a sensitivity factor that depends only on the accelerating voltage and the X-ray detector configuration. When the elements in the specimen do not have large differences in their X-ray mass absorption coefficients, or when the specimen is very thin, corrections for X-ray absorption may be negligible and need not be applied to obtain an accuracy of 10-2OYo relative to the amount of element present. This modest level of analysis is still remarkable when it is realized that it may be obtained from regions of a specimen about 5 nm in diameter. To obtain quantitative results in the 5 1 0 % range, an absorption correction should be applied using an estimate of the specimen thickness from CBED, EELS, or another method.55 Using EDS methods, elemental detection is possible down to a fay % wt. for elements having atomic numbers Z e 11 and down to 0.1-0.5 wt % for elements having Z > 11.

Microanalysis by Electron Energy-Loss Spectrometry

Electrons in the incident beam suffer inelastic collisions with atoms in the specimen; the effect of these collisions may be detected by measuring the energy of the primary electron after it has traversed the specimen. To observe a useful signal above background, very thin specimens (about 10-50 nm thick) must be used. Of the several inelastic events possible, the most usefd for elemental analysis is the inner-shell ionization event that leads to characteristic X-ray and Auger electron emission. The shape of the spectrum at these characteristic inner-shell ionization energy losses is similar to the X-ray absorption edge. The signal intensity under the edge and above background can be related to the amount of the element in the ~pecimen.~ This microanalysis method is somewhat less accurate than EDS X-ray analysis because the ionization cross section, which is needed to convert the collected intensity to chemical composition, is often not well known. Details in the EELS spectrum reveal bonding information and information about the dielectric constant from regions of the specimen as small as 0.5 nm in diameter. Detection limits are similar to EDS, but the method is best applied to the K-edges for light elements from lithium to fluorine. 166


Chapter 3

I Annular dark-field STEM image of individual gold atoms on a very thin carbon film: (a) individual gold atoms appear as bright spots; and (b) higher magnification image showing a single gold atom. The scan lines are caused by the 0.25-nm electron beam traversing the gold atom about 15 times. (Courtesyof M. Isaacson)

Figure 4

Examples of Common Analysis Modes

The major STEM analysis modes are the imaging, diffraction, and microanalysis modes described above. Indeed, this instrument may be considered a miniature analytical chemistry laboratory inside an electron microscope. Specimens of unknown crystal structure and composition usually require a combination of two or more analysis modes for complete identification. ConventionalBright-Field Imaging

Bright-field STEM images provide the same morphological and defect analysis typical of TEM images, such as particle sizing, interface analysis, and defect analysis (see Figure 3a). While the contrast may differ from TEM for thin crystalline materials, a dedicated STEM instrument using a field emission gun produces images that are similar enough to use the same image interpretation rationale developed for conventional TEM analysis. Annular Dark-field Imaging

The annular dark-field detector of the field-emission STEM (see Figure 2) provides a powerful high-resolution imaging mode that is not available in the conventional TEM or TEM/STEM. In this mode, images of individual atoms may be obtained, as shown in Figure 4 (see Isaacson, Ohtsuki, and Utlaut'). Some annular dark-field 3.4

STEM

167

Si02

Ge

Si

Figure 5

Images of a thin region of an epitaxial film of Ge on Si grown by oxidation of Ge-implantedSi: (a) conventional TEM phase contrast image with no compositionalinformation;and (b) high-angledark-fieldSTEM image showing atomically sharp interfacebetween Si and Ge. (Courtesy of S.J. Pennycook)

detectors have been modified to collect only the electrons that scatter into angles greater than 80 milliradians By collecting only the high-angle Rutherford-scattered electrons, images may be obtained that contain compositional information, as well as atomic-level image detail' (see Figure 5). Point Group and Space Group Determination

Crystallographicstructure determination is generally considered the realm of X-ray diffraction. However, three-dimensional sampling of the crystal by CBED allows first-principle determinations of point groups (crystal classes) for crystals as small as 100 nm, which could never be accomplished by X-ray methods. Figure 6 shows CBED patterns of a phase at the triple point of aluminum nitride grains containing AI,N, and 0.These patterns helped to determine the point group and space group of this aluminum oxynitride spinel phase as m3m and Fd3m, respectively.'* Microanalysis

Microanalysis of specimen regions in the nm-size range is one of the strongest reasons to use STEM. Figure 7 shows X-ray and EEL spectra taken simultaneously from the same area of a thin catalyst specimen. In each case the energy position of the peak or edge provides element identification, whereas the intensity above background for each peak and edge allows quantitative assessment of the composition. The statistics for these data are atypical owing to an intentionally short acquisition time of a few seconds. Figure 8a shows the statistics for a more typical X-ray spec168


Chapter 3

a

b

CBED patterns of aluminum oxynitride spinel along the [OOl] direction. Symmetries in the patterns contributed to the determination of the point group and space group: (a) whole pattern showing 1st Laue zone ring; and (b)0th order Laue zone. Both patterns show a fourfold rotation axis and two mirror planes parallel to the axis. (Courtesy of V. P. Dravid)

Figure 6

trum collected over 100 s. Note the low-energy carbon and oxygen peaks collected with a windowless X-ray detector. Phase Identification

Knowledge of the elements in an unknown phase, as determined by EDS or EELS, usually does not permit identification of the compound. However, this elemental information may be used in combination with interplanar spacing information (d-values) from one or more CBED patterns (or even the SAD patterns mentioned in the article on TEM) to render a positive identification. The task of searching all inorganic compounds to make a positive identification is now easier because of a new index to the JCPDS-ICDD Powder Diffraction File and the NIST Crystal Data File called the Elemental and Lattice Spacing Index.13 The elements determined from EDS or EELS analysis are used for the primary search, which places all possible compounds on one or two pages for confirmation by matching to the ten largest interplanar spacings listed for each entry. Compositional Imaging

Elemental distributions in a thin specimen may be obtained at high resolution from any properly prepared solid specimen using either EDS or EELS signals. These images are sometimes called elemental maps. Elemental images usually collected digitally by setting a region of interest in the spectrum for each element and storing the counts collected in these windows as a function of electron beam position (stor3.4

STEM

169

a

7

8

9

10

11

X-Ray Energy (keV)

b

800

900

1000

1100

1200

Electron Energy Loss (eV) Figure 7

Microanalysis of a CuO/ZnO methanol synthesis catalyst with a field-emission STEM: (a) EDS data showing Cu and Zn K-lines; and (b] EELS data showing Cu and Zn Ledges with dotted lines indicating background levels. Spectra were taken simultaneously from a 2-nm diameter area. Signal intensities above background show that approximately the same relative amounts of Cu and Zn were measured by each method.

age at each image pixel), as shown in Figure 8b. An image of the X-ray background signal was collected separately ro determine that the distribution of sulfur in Figure 8 is real.14 Since the electron probe size is governed by the need for a large probe current (about 1 nA), image magnification, pixel density, and counting rime 170


Chapter 3

6wi,lL 100

400

200

c.

Figure8

X-ray elemental imaging in a field-emission STEM: (a) EDS data of Pd /Ce /alumina catalyst particle poisonedwith SO,; and (b) 128 X 128 digital STEM imagesformed using X-ray counts collected at each image pixel for aluminum, palladium, cerium, and sulfur. (Courtesy of North-Holland Publishers~’~

per pixel must be adjusted for the type of STEM instrument used. For example, if the pixel density is 128 x 128 with a counting time of 100 ms per pixel, a compositional image may be obtained in 28 min. However, for a thermionic source in a TEM-STEM system, where the counting time per pixel must be increased to 3 s per pixel, the total frame time for a similar image is about 14 hours. When elements are present in high concentration (>2Owt %), the resolution of the X-ray elemental image can be c 5 nm. Elemental images using EELS signals can be of even higher resolution, although more computation is necessary to subtract the background. Sample Requirements

Specimens suitable for imaging in TEM are usually acceptable for STEM imaging and analysis. The principal methods for producing suitable thin specimens are electropolishing (metals only), ion-beam thinning (all hard materials), and ultramicrotomy (polymers, some metals, and small hard par tide^).'^ In the first two cases the specimen is thinned to perforation, and analysis takes place around the edge of the hole. Ultramicrotomed specimens are relatively uniform in thickness, but the fragile thin sections must be supported by a specimen grid. Hard, brittle materials also may be crushed to a powder and dispersed on a carbon film supported on a specimen grid, as in Figure 3. For microscopy of layered materials it is usehl to dice the specimen to reveal a cross section of the layers before ion-beam milling. Precipitates in a metal matrix may be extracted using the extraction replica technique. Microanalysis often places special constraints on the preparation of thin specimens beyond the general requirement to be transparent to 100-keV electrons. 3.4

STEM

171

CBED usually requires analysis along a particular crystallographic direction, and although specimen-tiltingfacilitiesare available in the microscope, ofien it is usefbl to orient the specimen before specimen preparation so that the thinned direction is along a crystal direction of interest. For high-quality microanalysis by EDS or EELS, the thin specimen must be free of surface films containing elements redistributed from the bulk or from contamination sources that would change the measured through-section composition. Ion-beam milling for short times usually removes such films deposited by electropolishing; however, ion-beam milling for long times may introduce elemental redistribution by differential sputtering. Specimens prepared by ultramicrotomy, extraction replication, or crushing generally do not have these compositional artifacts. Occasionally microanalysis is performed on specimens prepared by one of the latter methods while images of the same material might be obtained from electropolished or ion-beam milled specimens. Artifacts

The major artifact typical of STEM imaging is a buildup of hydrocarbon contamination under the electron beam. This contamination appears in the bright-field image as a dark area in the shape of the scanning raster or as a dark spot if the beam has been stopped for microanalysis. Besides obscuring features in the image, contamination layers can absorb X rays from the lighter elements and increase the background in EELS analysis. Contamination can be reduced by heating the specimen in vacuum before examination, by cooling the specimen during analysis, and by improving the vacuum in the specimen chamber to better than lo4 Pa (about 1O* torr). In addition to cleanliness (contamination effects), surface morphology and the alreration of composition during specimen preparation can cause serious artifacts in microanalysis. In some older instruments, the microscope itself produces undesirable high-energy X rays that excite the entire specimen, making difficult the accurate quantitation of locally changing composition. Artifacts also are observed in the EDS X-ray spectrum itself (see the article on EDS). Conclusions

STEM can provide image resolution of thin specimens rivaling TEM,but in addition can provide simultaneous crystallographic and compositional analysis at a higher spatial resolution than any other widely-used technique. Any solid material may be examined, provided that a specimen can be prepared that is less than about 100 nm in thickness. Future developments of t h i s instrumentation indude field emission electron sources at 200-300 kV that will allow better elemental detectabilityand better spatial resolution. Multiple X-ray detectors having large collection angles will also improve elemental detectability in X-ray microanalysis. The higher accelerating 172


Chapter 3

voltages should allow EDS X-ray and EELS compositional measurements to be made on specimens of the same thickness, instead of requiring a much thinner specimen for EELS. Electronic capture of digital diffraction pattern images should make some automation of diffraction pattern analysis possible. Under complete digital control automated experiments will be possible that cannot be accomplished manually. Related Articles in the Encyclopedia

TEM, EDS, EELS, and SEM References 1

Introduction to Anafyticaf Electron Microscopy (J. J. Hren, J. I. Goldstein,

and D. C. Joy, eds.) Plenum, New York, 1979. Somewhat dated text that is still useful for certain chapters including those of Cowley, Steeds, and Isaacson, et al. 2 Quantitative Microanalysis with High Spatial Resolution. (G. W. Lorimer, M. H. Jacobs, and I? Doig, eds.) The Metals Society, London, 1981. Research papers giving results from many materials. 3 D. B. Williams. Practical Analytical Ekctron Microscopy in Materials Science. Philips Electronic Instruments, Mahwah, NJ, 1984. Concise textbook on CBED, EDS, and EELS with a pronounced “how-to” flavor. 4 Principles ofAnalytica1 Electron Microscopy. (D. C. Joy, A. D. Romig Jr, and J. I. Goldstein, eds.) Plenum, New York, 1986. An updated version of Reference. 1. 5 R. E Egerton. Electron Energy Loss Spectroscopy in the Electron Microscope. Plenum, New York, 1986. The principle textbook on EELS. 6 High Resohtion Transmtssion Electron Microscopy andhsociated Techniques. (I? R. Buseck, J. M. Cowley, and L. Eyring, eds.) Oxford University Press, New York, 1988. A review covering these techniques in detail (except Xray microanalysis) including extensive material on high-resolution TEM. 7 A. V. Crewe, J. Wall, and J. Langmore. Science. 168,1338, 1970. The classic first attempt to image single atoms with the STEM. a J. Mansfield. Convergent Beam Electron Dzfiaction ofAlloy Phases. (by the Bristol Group, under the direction of J. Steeds, and compiled by J. Mansfield) Hilger, Bristol, 1984. This book is an atlas of CBED patterns that may be used to identifi phases by comparing published patterns with experimental patterns. 3.4

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173

s B. E Buxton, J. A. Eades, J. A. Steeds, and G. M. Rackham. Phil. Trans. R. Soc. A. 281, 171, 1976. This paper outlined point group determination for the first time, but the major conclusions are also summarized in Williams (op. cit.). IO G. Cliff and G. W. Lorimer. J. Microscopy 103,203, 1975.This paper summarizes the Cliff-Lorimer analysis technique, but more complete reviews of the method may be found in References 1-4. 11 S. J. Pennycook. EMS4 Bulletdn. 19,67, 1989. A summary of compositional imaging using a high-angle annular dark-field detector in a field emission STEM instrument published by the Electron Microscopy Society ofAmerica, Box EMSA Woods Hole, MA 02543. 12 V I? Dravid, J. A. Sutli& A. D. Westwood, M. R Notis, and C. E. Lyman. Phil. Mag. A. 61,417, 1990. An example of how to practically determine the space group of a phase. 13 JCPDS-KDD Elemental and Lattice Spacing Index ( 1990).This index is available from JCPDS-International Centre for Diffraction Data, 1601 Park Lane Swarthmore, PA 1908 1. 14 C. E. Lyman, H. G. Stenger, and J. R. Michael. Ultramicroscopy.22, 129, 1987. This paper demonstrates high-resolution compositional imaging with the field-emission STEM. 15 Specinaan Preparationfor Transmission Electron Microscopy of Materiah (J. C. Brauman, R. M. Anderson, and M. L. McDonald, 4 s . ) MRS Symp. Proc vol. 11 5, Materials Research Society, Pittsburg, 1988. This conference proceedings contains many up-to-date methods as well as references to books on various aspects of specimen preparation.

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3.5

EPMA Electron Probe X-Ray Microanalysis' DALE E. N E W B U R Y

Contents

Introduction Physical Basis of EPMA Measurement of X-Rays Qualitative Analysis Quantitative Analysis Compositional Mapping Conclusions

Introduction

Electron Probe X-Ray Microanalysis (EPMA) is a spatially resolved, quantitative elemental analysis technique based on the generation of characteristic X rays by a focused beam of energetic electrons.'-3 EPMA is used to measure the concentrations of elements (beryllium to the actinides) at levels as low as 100 parts per million (ppm) and to determine lateral distributions by mapping. The modern EPMA instrument consists of several key components: 1 An electron-optical column capable of forming a beam ranging in diameter from nm to pm and carrying a current ranging from PA to fi 2 An energy-dispersiveX-ray spectrometer and at least one wavelength-dispersive X-ray spectrometer 3 An optical microscope for precise positioning of the specimen relative to the Xray spectrometers 4 A vacuum system capable of operating at pressures ranging from 104 to lo4 Pa 1. Note: This artcle is a contribution of the United States Government and is not subject to copyright.

3.5

EPMA

175

Transiton of Outer Shell Electron

Characteristic X-ray Emission

Incident Primary Eleclron, E

,

--0 Ejected Orbital Electron

Figure 1

5

Schematic of inner shell ionization and subsequent deexcitationby the Auger effect and by X-ray emission.

A computer system to control the beam, spectrometers, specimen stage, and quantitative data processing.

The electron-optical performance of the EPMA system is indistinguishable from that of a conventional scanning electron microscope (SEM); thus, EPMA combines all of the imaging capabilities of a SEM with quantitative elemental analysis using both energy- and wavelength-dispersiveX-ray Physical Basis of EPMA

The physical basis of electron probe microanalysis is the generation of characteristic X rays by bombarding a solid with energetic electrons. Interactions of energetic electrons with matter include elastic scattering, which causes significant angular deviation of the electron trajectories, and inelastic scattering, which reduces the energy of the electrons and sets a limit to their range. Mechanisms of inelastic scattering include the production of secondary electrons, inner shell ionization, bremsstrahlung (continuum) X rays, long-wavelength electromagneticradiation in the ultraviolet, visible, and i n k e d regions, electron-hole pairs, lattice vibrations (phonons), and electron oscillations (plasmons). The rate of energy loss depends on the electron energy and the target composition, and is typically 0.1-10 eV/nm. Inner shell ionization, illustrated in Figure 1, occurs when the beam electron ejects a bound atomic electron by transferring sufficient energy to the atom to exceed the critical excitation (or binding) energy E, of an atomic electron. The resulting atomic vacancy is filled by a transition involving one or more outer shell electrons. The energy difference between shell levels can be manifested as an emitted X-ray photon, or the energy can be transferred to another outer shell electron, which is ejected as an Auger electron. Because the energy of the atomic shells is sharply defined, the emitted X ray or Auger electron has a precisely defined energy and is therefore characteristic of the atom species originally ionized by the beam electron. 176


Chapter 3

For atoms having an atomic number greater than 10, the electron filling the inner shell vacancy may come from one of several possible subshells, each at a different energy, resulting in families of characteristic X-ray energies, e.g., the Ka,p family, the La, p, y family, etc. The beam electrons also produce X rays when they undergo deceleration in the Coulombic field of the specimen atoms, losing energy and emitting X rays known as “braking” radiation, or brmstrahlung. Since this energy loss can take on any value from 0 to the entire energy carried by the high-energy electron, the bremsstrahlung X rays form a continuum background spanning all energies. Bremsstrahlung X rays are indistinguishable from characteristic X rays that may occur at the same energy. Statistical fluctuations in this bremsstrahlung background form the eventual limit that determines the ultimate sensitivity of electron probe X-ray microanalysis. The critical parameter for X-ray generation is the overvoltage U =&/E,,where & is the incident beam energy. The intensity of characteristicX rays is given by: lCh = aiB( u- 1)”

where a is a constant, is the beam current, and the exponent n is approximately 1.5. Equation (1) demonstrates that the analyst must choose a beam energy that exceeds the critical excitation energy for the species to be analyzed. In general, a value of U>2 is required to achieve adequate efficiencyin the production ofX rays. The volume of analysis, i.e., the diameter and depth of the analyzed region, is limited by a combination of the elastic and inelastic scattering.The maximum depth of the interaction volume is described by the Kanaya-Okayama electron range:

where Z i s the atomic number, A is the atomic weight (g/mole), p is the density (g/cm3), and 4 is the incident beam energy (kev). The maximum width of the interaction volume is similar in value to the range. When the production of X rays is considered, the range is reduced because X rays for a particular element cannot be produced below E; for that element. Equation (2) is modified to reflect this condition:

(-

)

Rx (pm) = 0.02764 (

- E:.67)

(3)

z0.89P Monte Carlo electron trajectory simulations provide a pictorial view of the complex electron-specimen interaction. As shown in Figure 2a, which depicts the interac3.5

EPMA

177

Copper E, = 30 keV Tilt =Oo Single scattering

a

05pm

b

-

.-

.

Figure 2

178

(a) Monte Carlo simulation of electron trajectories in copper, beam energy 30 keV. (b) Corresponding sites of K-shell ionization.


Chapter 3

-*

7 0.2

Cu

- 5% AI; E

** 1

0

-1 0.15 €

0.1

I=

0.05

L

2

cu AI

. *.. . * . * . ..t

A

E

20 keV

0

i 0

..

***:. I

I

1-

0.1

0.4

0.8

0.8

1

Depth (mlcrometerc)

Figure 3

Depth distribution of generation of Cu KOr X rays for an incident beam energy of 20 keV, and the effect of absorption.

tion volume of a beam incident perpendicularly on a flat specimen of copper, the interaction volume has dimensions on the order of pm. Generally, the spread of the electrons through the specimen, not the diameter of the incident probe, is the limiting factor in determining the spatial resolution of analysis. A focused beam carrying sufficient electron current for analysis can be obtained with a diameter that is generally a factor of 5-10 smaller than the dimension of the interaction volume given by Equation (3). For example, the range for production of Cu Ka X rays at an incident beam energy of 20 keV in copper is approximately 1 pm, while a beam diameter of 100 nm (0.1 pm) would carry sufficient beam current for analysis. The production of X rays within the interaction volume is illustrated in Figure 2b. An important concept related to the interaction volume is the depth distribution of X rays, the so-called $(pz) function shown in Figure 3. This function forms the basis for the development of matrix correction procedures in the quantitative analysis protocol described below. Measurement of X Rays

The analyst has two practical means of measuring the energy distribution of X rays emitted from the specimen: energy-dispersive spectrometry and wavelength dispersive spectrometry. These two spectrometers are highly complementary; the strengths of each compensate for the weaknesses of the other, and a well-equipped electron probe instrument will have both spectrometers. Energy Spectrometry (EDS) uses the photoelectric absorption of the X ray in a semiconductor crystal (silicon or germanium), with proportional conversion of the X-ray energy into charge through inelastic scattering of the photoelectron. The quantiry of charge is measured by a sophisticated electronic circuit linked with a computer-based multichannel analyzer to collect the data. The EDS instrument is 3.5

EPMA

179

purely a line-of-sight device; any X ray emitted from the specimen into its solid angle of collection can be measured. T o reduce thermal noise, the detector crystal and preamplifier are cooled to liquid nitrogen temperatures. For application to conventional electron-optical columns with a pressure on the order of IO4 Pa, a thin window of beryllium, diamond, boron carbide, or a metal-coated polymer is used to protect the crystal from condensing water vapor from the microscope environment. The window material, the gold surface electrode, and an inevitable “dead layer” of partially active semiconductor just below the electrode act to absorb X rays. With optimum low-mass window materials like diamond or boron carbide, the EDS can measure X rays as low in energy as the Be K level (0.109 kev). Wavelength Spectrometry (WDS) is based upon the phenomenon of Bragg diffraction of X rays incident on a crystal. The diffraction phenomenon is described by the expression:

where h is the wavelength equivalent of the X-ray energy E h (nm) = 1.2398/E (kev), dis the crystal spacing for the diffractingplanes: and 8B is the angle at which constructive interference occurs (i.e., the Brag%angle). The WDS instrument is mechanical in nature and is a focusing device (see Figure 1). The X-ray source at the specimen, the diffracting crystal, and the gas counter to detect the X rays must follow a specific geometry (a Rowland circle) to satisfy Equation (4); accurate positioning is ensured by incorporating an optical microscope into the electron probe. To detect a wide energy range, different crystals with various d spacings are mounted on a mechanical turret, and are selectable under computer control. EDS and WDS spectra of a multielement glass, shown in Figure 4, illustrate some major strengths and weaknesses: 1 Spectral acquisition. An EDS instrument can measure any X-ray photon that reaches the detector, and thus can continuously view the entire X-ray spectrum (0.1-20 kev), while the WDS instrument must be mechanically scanned with several crystal changes to acquire a complete spectrum over that same range. In Figure 4, for equivalent sensitivity, the WDS spectrum required approximately a factor of 10 greater acquisition time. When an unknown is examined, it is highly unlikely that a complete qualitative WDS scan will be made at every location chosen for analysis. Since EDS is always capable of measuring a complete spectrum, unexpected major and minor constituents can always be detected. For survey work of major and minor constituents in unknowns, EDS therefore has a significant advantage for rapid qualitative analysis. 2 Spectral resolution. Consider a Mn Ka photon, which has a natural peak width (the full width at half maximum, or FWHM) of approximately 1 180


Chapter 3

Energy

Wavelength

Figure 4

Comparison of EDS and WDS spectra from a complex, mukielement glass. [Courtesyof Charles Fiori, National Institute of Standards and Technology)

eV. In WDS, the diffraction process, diffraction crystal imperfections,and the mechanical construction degrade this peak to a FWHM of approximately 10 eV. In EDS, the statistics of charge carrier creation degrade the peak to a FWHM of approximately 140 eV The effect of spectral resolution can be seen readily in Figure 4, where a single peak in the EDS spectrum is actually composed of Mn K a (kev) + Ba La2,3. This double peak is easily resolved using M D S into two peaks that differ in relative intensity by a kctor of 10. Although powerful methods for mathematical spectral deconvolution of peaks can be applied to EDS spectra, deconvolution is likely to produce false solutions when the peaks are separated by less than 50 eV, when the peaks are sharply different in relative intensity, and when 3.5

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181

significant statistical fluctuationsexist in the spectral data, which is usually the situation in practical analysis. A classic exampleof an EDS interference situation encountered in practical analysis is that of S Ka,p (2.307 kev), M o La, p (2.29 kev), and Pb Ma,j3 (2.346 kev); this interference situation is easily resolved by WDS. 3

Peak-to-background ratio and limits of detection. The peak-to-background ratio, which is a direct consequence of the spectral resolution, plays a major role in determining the limits of detection. A peak is more sharplydefined with WDS than with EDS. The peak-to-background ratio is proportionally higher, since the intensity due to the characteristic radiation is spread over a smaller energy range of the X-ray continuum background. In practical analysis situations, the limit of detection for WDS is 100 ppm while that for EDS is approximately 0.1 % wt. (1000 ppm) for elements having an atomic number > 11 (sodium).The detection limits are substantiallypoorer for light elements because of the high absorption losses in the specimen and spectrometercomponents. Depending on the matrix, the detection limit for light elements may be as high as 1-10% wt.

4

Dead time and limiting count rate. The pulse-processing time (dead time) is on the order of 10-50 ps for EDS and 0.5-1 ps for WDS. Because the WDS makes use of the diffraction effect to disperse the X rays before the detector, the limiting count rate applies to a single X-ray peak of interest; limiting count rates of lo5 cps are possible and trace constituents can be measured with no dead time fiom major constituent peaks. The EDS instrument, which has a limiting count rate of approximately 2.5 x lo4 cps, must process every X-ray photon in the range from the low-energy cutoff (approximately 100 ev) to the beam energy. The EDS dead time applies to the total spectrum rather than to a single peak of interest, making trace constituent measurement much more difficult.

5

Spectrometer positioning. The WDS instrument is a focusing optical device, with an ellipsoid of volume in focus fbr uniform X-ray transmission. The long axes of the ellipsoid have dimensionson the order of 100 to 1000 prn, while the short axis is approximately 10 pm. Precise positioning is vital if consistentX-ray intensity measurements are to be obtained. On a formal electron probe, this positioning is ensured by incorporating an optical microscope with its shallow depth of field and mechanically translating the specimen stage. The EDS instrument is a nonfocusing device, and it is therefore not critical to position it accurately relative to the specimen.

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Qualitative Analysis

Qualitative analysis is generally assumed to be a trivial procedure consisting of matching the observed peaks to tabulated energies. Identification of peaks of major constituents is possible with a high degree of confidence, but misidentification of low-intensity peaks from minor and trace constituents is very much a possibility unless a careful strategy is followed. A good qualitative analysis strategy consists of first identifying the principal peaks of a major constituent and then locating all associated low-intensity peaks, including other X-ray h i l y members, as well as all spectral artifacts, such as escape and sum peaks for EDS; for WDS, multiple-order lines--n = 2, 3, 4, etc., in Equation (4)-and satellite lines must be located. Only then should remaining peaks be identified as minor and trace constituents. Generally, the confidence with which a minor or trace constituent can be identified will be reduced because fewer peaks can be recognized above background. Absolutely no spectral smoothing should be applied in EDS spectra, since this introduces false “peaks” due to statistical fluctuations. Quantitative Analysis

The keystone of practical quantitative electron probe microanalysis is Castaing’s first approximation, which relates the concentration for a constituent in the unknown to the concentration in a standard in terms of the ratio of X-ray intensities generated in the target:’

ci

Ii“

where Cis the mass concentration, IGis the generated characteristic peak intensity (corrected for dead time, peak overlaps, and background), i denoas a specific constituent, and * denotes the standard, such as a pure element. What is actually measured is the ratio of emitted intensities 4!/ 50 pm, to contain the electron-excited volume as well as the full range of X-ray induced secondary fluorescence. No chemical polishing or etching can be tolerated because of the danger of modifying the near-surface composition through selective leaching.

5

The specimen must be homogeneous throughout the interaction volume sampled by the beam, since X rays of different energies originate from different depths.

6

The atomic number of the analyzed species must be 11 (sodium) or higher. For light elements from beryllium to fluorine, the low energy of the characteristic X rays leads to high absorption situations and an absorption correction factor exceeding2. The mass absorption coefficientsfor the X rays of these elements are poorly known, so that large relative errors exceeding 10% are often encountered. Consequently, in fully oxidized systems, oxygen is frequently calculated indirectly by means of assumed stoichiometry for the cation species.

If all constituents are separately measured, or are calculated by stoichiometry, as in the case of oxygen, then the analytical total, which is the sum of all elements plus any calculated by stoichiometry, conveys useful information. If the analytical total is above 102%, a possible mistake in the measurement of the electron dose or an unexpected change in spectrometer parameters is likely. If the analytical total is significantly below 98%, an additional constituent is likely to have been missed. When multielement standards similar in composition to the unknown are available, a purely empirical mathematical procedure, the Bence-Albee or Ziebold-Ogilvie method, can be applied. This procedure is often used in the analysis of minerals for which close standards exist. The “standardless” approach attempts to apply first principles descriptions of X-ray production to the calculation of interelement relative sensitivities. Several of the key parameters necessary for first principles calculations are poorly known, and the accuracy of the standardless method ofcen suffers when different X-ray families must be used in measuring several elemental constituents in a specimen.

7

Special Cases of Quantitative Analysis

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Particles, rough surfaces, inclusions, layered film materials, and films on substrates represent special cases of considerable technological importance where the conventional quantitative analysis procedures Ed. In such materials, shape and dimensions can influence the interaction of the beam electrons and the production of X rays. Several different strategies have been developed for dealing successfullywith such geometric These methods range from simple approaches, such as normalization of the analytical total to compensate for particle effects, to complex mathematical treatments, including theoretical or empirical modifications of the ZAF correction factors and rigorous Monte Carlo electron trajectory simulation of the exact geometrical boundary conditions of the specimen. Films on substrates, including multilayers, can be analyzed with corrections calculated by the g(pz) m e t h ~ dFor . ~ particles and rough surfaces, errors in excess of 100% relative may be encountered with calculations uncorrected for geometricaleffects;with simple normaliition the maximum errors will generally be less than 25% relative. When the feature of interest is an inclusion contained in a matrix of different composition, where the lateral and depth dimensions of the inclusion are less than the X-ray range, quantitative and even meaningfd qualitative analysis becomes impossible, since the interaction volume simultaneously samples both the inclusion and the surrounding matrix. In such cases, it may be necessary to mechanically or chemically separate the inclusions, and to analyze them as freestandingparticles. CompositionalMapping

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositionalvariations, or with the associated optical microscope. Often, more detailed information is needed on the distribution of a constituent. The technique of X-ray area scanning, or dot mapping, can provide a qualitative view of elemental distributions. As the beam is scanned in a raster pattern on the specimen, a cathode ray tube scanned in synchronism is used to display a fill white dot whenever the X-ray detector ( W D S or EDS) detects an X ray within a certain narrow energy range. The pattern of dots is recorded on film to produce the dot map. Dot maps are subject to the following limitation^:^ 1 The information is qualitative in nature. The area density of dots suggests local concentration differences, but the count rate at each point, which is hndamental informaaon requiredfor quantitation, is lost. 2 Because of count rate performance and peak-to-background, WDS is preferable to EDS for mapping, particularlyfbr minor and trace constituents. 3.5

EPMA

'I87

Figure 5

X-ray area scan (dot map) showing the distributionof zinc at the grain boundaries of copper.

Mapping of major constituents can be carried out in approximately 15-30 minutes of scanning per image. Minor constituents require 0.5-3 hours, and trace constituents require 3-10 hours. An example of a dot map of zinc at concentrations in copper as low as 1% is shown in Figure 5; 6 hours of scan time was needed to produce a dot map at this level. While low concentrations can be mapped against a background of 0 concentration, as shown in Figure 5, dot mapping has poor contrast sensitivity. It would be difficult to map a 5% concentration modulation above a general level of 50% because there is no provision for subtracting the influence of the dominant general signal. Since no background correction can be made, dot maps of minor and trace constituents are subject to possible artifacts caused by the dependence of the bremsstrahlung on composition, particularly with EDS X-ray measurement. Quantitative compositional mapping overcomes the limitations of dot mapping by recording in computer memory the count rates for all constituents of interest at all picture elements in the scan.* Complete quantitative procedures are then applied to the count rate arrays: dead time correction, background correction, standardization, and matrix correction. The resulting concentration arrays are presented as images on a digital display with a gray or color scale encoding the

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a

b

Figure 6

3.5

EPMA

(a) Quantitative compositional map of the distribution of zinc at the grain boundaries of copper; gray scale corresponds to the concentration range 010% wt.; superposed compositional profile (vertical scale, 0-4.040wt.) along the locus AB. (b) Contrast enhancement applied to emphasize low-concentration structure (0.196 wt. = 1000 ppm) at arrows; image width 100 pm.

189

concentration information. Typical dwell times per pixel are 0.1-2 s, which restricts detection limits in imaging to approximately 500 ppm with high beam currents and WDS detection; comparable accuracy to single point analysis can be achieved in the mapping mode. An example is shown in Figure Ga of a digital compositional map of the same area as Figure 5; concentration levels as low as 0.1 % wt. are visible. Digital compositional maps can be subjected to subsequent image processing to enhance features of interest. Contrast expansion of Figure Ga, shown in Figure Gb, reveals fine details in the zinc diffusion zone. This combination of images supported by complete numerical concentration d u e s at every pixel, including the measurement statistics, is a powerful tool for solving a wide range of problems. Conclusions

The electron probe X-ray microanalyzer provides extraordinary power for measuring the elemental composition of solid matter with lateral spatial resolution. The spatial resolution, limited by the spread of the beam within the specimen, permits pg samples to be measured selectively,with elemental coverage from boron to the actinides. By incorporating the imaging capability of the SEM, the electron probe X-ray microanalyzer combines morphological and compositional information. Several future trends are evident. The great utility of true compositional maps suggests that this mode of operation will replace the now antiquated dot maps. Greater use of computer-aided imaging and analysis will increase the efficiency of the instrument in applications where large quantities of data are required. Improved long-term stability will permit the long counting times required for routine application of trace analysis. Continued development of advanced correction methods will address special difficulties raised by complex specimens such as multilayers and inhomogeneousparticles. Lastly, the need for improved lateral and depth spatial resolution will lead to increased application of low beam energy analysis. Related Articles in the Encyclopedia

EDS, XRF, and SEM References I

R Castaing. Application ofEhctroon Probes to Local Chemicaland Ctystallo-

graphic Ambsis. Ph.D. thesis, University of Paris, 1951. z J. I. Goldstein, D. E. Newbury, I? Echlin, D. C. Joy, C. E. Fiori, and E. Lifshin. Scanning Ehctron Miuoscopy andX-RayMicroam&. Plenum, New York, 1981. References 2 and 3 are comprehensivetextbooks covering all

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aspects of electron probe microanalysis and the associated technique of scanning electron microscopy. 3

D. E. Newbury, D. C. Joy, I? Echlin, C. E. Fiori, and J. I. Goldstein. Advanced Scanning Electron Microscopy andX-Ray Microanalysis. Plenum, New York, 1986.

4

5

D. E. Newbury, C. E. Fiori, R. B. Marinenko, R L. Myklebust, C. R. Swyt, and D. S. Bright. Compositional Mapping with the Electron Probe Microanalyzer. Anal. Cbem. 1990,62, Part I, 1159A; Part 11, 1245A. G. E Bastin and H. J. M. Heijligers. Quantitative Electron Probe Microanalysis of Ultralight Elements (Boron-Oxygen). Scanning. 12, 225,1990.

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4 STRUCTURE DETERMINATION BY DIFFRACTION A N D SCATTERING X-Ray Diffraction, XRD 198 Extended X-Ray Absorption Fine Structure, EXAFS 214 Surface Extended X-Ray Absorption Fine Structure and Near Edge X-Ray Absorption Fine Structure, SEXAFS/NEXAFS 227 4.4 X-Ray Photoelectron and Auger Diffraction, XPDandAES 240 4.5 Low-Energy Electron Diffraction, LEED 252 4.6 Reflection High-Energy Electron Diffraction, W E E D 264 4.1 4.2 4.3

4.0

INTRODUCTION

This chapter contains articles on six techniques that provide structural information on surfaces, interfaces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, W E E D ) , or X rays in and electrons out (Surhce Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their “usual” form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are s&e sensitive. There are, however, ways to make XRD and EXAFS much more surface sensitive. For EXAFS this converts the technique into SEXAFS,which can have submonolayer sensitivity. The techniques can be broadly classified into two groups: those which directly identify the atomic species present and then provide structural information about the identified species from diffraction or scattering effects (EXAFS, SEXAFS, and XPD); and those which are purely diffraction-based and do not directly identify the atoms involved, but give long-range order information on atomic positions from 7 93

diffraction patterns (XRD, LEED, and RHEED). The latter group is only concerned with crystalline material and, for individual crystallinephases present, average unit cell dimensions, symmetries, and orientations are obtained directly from the diffraction patterns. Deviations from the average, i.e., defects of some sort, show up as a broadening of diffraction peaks, or, if they are periodic, as splitting of

PeaksXRD is the most widely used technique for general crystalline material characterization. Owing to the huge data bank available covering practically every phase of every known material (powder diffraction patterns), it is routinely possible to identify phases in polycrystalline bulk material and to determine their relative amounts from diffraction peak intensities. Phase identification for polycrystalline thin films, using standard equipment and diffraction geometries, is also possible down to thicknesses of 100A. For completely random polycrystallinethin films relative amounts are also easily determined. Once preferred orientations occur (texturing) this gets more difficult, requiring the collection of much more data or the introduction of more sophisticated equipment with different diffraction geometries so that the orientations can be “seen” effectively. These diffraction geometries include Grazing IncidenceXRD (GIXRD), in which case the X-ray probing depth is greatly reduced. This has the effect of greatly improving surfice sensitivity and allowing a depth profiling mode (50 A to microns) by varying the incidence angle. When coupled to a synchrotron radiation X-ray source (to produce an intense, parallel X-ray beam) monolayer sensitivity can be achieved by GIXRD. GIXRD can therefore be used fbr the extreme situation of surface structure, or epitaxial relationships at the interfaces of films, where atomic positions can be determined to an accuracy of 0.001 A in favorable cases. Besides phase identification XRD is also widely used for strain and particle size determination in thin films. Both produce peak broadenings, but they are distinguishable. Compared to TEM, XRD has poor area resolution capability, although by using synchrotron radiation beam diameters of a few pm can be obtained. Defect topography in epitaxial films can be determined at this resolution. Since LEED and M E E D use electron, instead of X ray, diffraction, the probing depths are short and surface crystallographicinformation is provided. Vacuum is also required. LEED uses normal incidence, with electron energies between 10 and 1000 eV, whereas RHEED (5-50 keV) uses grazing incidence and detection. The grazing geometry restricts RHEED information to the top few atomic layers, even though it is intrinsicallyless surface sensitive than the lower energy LEED. In both cases diffraction comes from two-dimensional rows of atoms, compared to the three-dimensional planes of atoms in XRD. In LEED the difiaction pattern (electron intensity “spots” projected back onto a phosphor screen; cf., an X-ray backreflection h u e diffraction photograph) directly reveals the size and shape of the unit cell of the outermost, ordered atomic layer of single crystal material. To establish the locations of the atoms within the unit cell and the separation of the surhce 194

ELECTRON/X-RAY DIFFRACTION

...

Chapter 4

plane from the bulk requires detailed measurements of the diffraction peak intensities versus the incident electron energy (wavelength), plus a comparison to a complex theoretical treatment of the electron scatteringprocess in the material. Because of this only relatively few laboratories use LEED this way to determine the structures of clean, terminated bulk surfaces (which are often different from the bulk) and ordered adsorbed overlayers. The major use is to qualitatively check that single crystal surfaces are “well-ordered” and to determine the unit cell symmetries and dimensions of adsorbed overlayers. In analogy to the three-dimensional situation with XRD, disorder in the surfice plane broadens LEED diffraction spots, and, if periodic, splits them. Information on finite island sizes (cf., particlesize by XRD), strain, step densities, etc., can be obtained from detailed studies of the peak line shapes. Owing to its grazing angle geometry it is more complex, in general, to extract unit cell parameters from RHEED. If, however, the incident beam is aligned along a major surface crystallographicdirection, this can be achieved fairly simply. Again, surface disorder translates to diffraction spot broadening. In practice W E E D is used primarily to monitor in situ the nucleation and growth of epitaxial films (e.%., MBE). The grazing geometry leaves the surface very “open”for the deposition process. In addition it makes RHEED very sensitive to surface roughness effecn, such as island formation, since the reflected beam is actually transmitted through island “asperities,”giving rise to new diffraction features. In standard form neither LEED nor RHEED have much spatial resolution, beam spot sizes being a fraction of a mm. Microscopic modes exist, however, using electron beam columns. They are Low-Energy Electron Microscopy, LEEM, and ScanningReflection Electron Microscopy, SREM. Neither technique is discussed here, since they are too specialized. In EXAFS a tunable source of X rays (supplied by a synchrotron facility) is used to scan energies from below to above the Binding Energy, BE, of electron core levels for any atom present. An increase in absorption associated with excitation of that electron occurs at the BE. Detection of such absorption “edgejumps” (usually measured using transmission through many thousands of angstroms of material) directly identifies the atoms present. Above the absorption edge weak periodic oscillarions in the absorption strength occur due to interferences between the outgoing photoelectron wave from the absorbing atom and backscatteringof this wave from neighboring atoms. The oscillations thus contain local radial distribution information (within -5 A of the absorbing atom). The frequency (or frequencies) of the oscillations relate to the distance of the surrounding atomic shell (or shells); the amplitudes relate to the number and types of surrounding atoms. If multiple sites of the specific element concerned are present a superimposition of oscillation frequencies is obtained that can be difficult to analyze, but for single site situations accuracies to 0.03 A in bond length can be achieved using model compounds of known structure. Since local order is probed, the material need not be crystalline. High-2 elements are more easily studied than low-2 elements, because the signal

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195

strengths are higher and because, in general, the regions above their higher energy core level BEs are clear of other interfering absorption features over a large energy range. In bulk materials heavy elements can be detected and studied down to around 100 ppm. Also, for X-ray energies above -5 keV samples can be studied under ambient pressures instead of in vacuum. The technique is widely used for bulk catalysts and in biological materials. If the emitted electron intensity resulting from the core-level excitation process is used to detect the X-ray absorption process (in reflection), the bulk EXAFS method is turned into the surface-sensitive SEXAFS technique, where the depths probed are only a few atomic layers. Surface structures (absorption sites, bond lengths) for atoms absorbed on single crystal surfices have been determined this way. The advantage compare to LEED is that the information is element specific, and long-range order of the adsorbate is not required. Accurate information is only obtainable, however, when the atom occupies a single unique site. Often C, 0,N, S, etc., are studied. These light elements have core levels in the low-energy “soft X-ray” region so that vacuum techniques are required. For monolayer surface studies this is always true, anyway. It is also possible to use X-ray fluorescence as a detection scheme, instead of electrons. In this case .the probing depth is extended to many hundreds of angstroms for the light elements, and deeper for the heavy elements. For molecules adsorbed on surfaces strong features often appear in the spectrum at or very near the absorption edge-jumps (termed Near Edge X-ray Absorption Fine Structure, NEXAFS.) These are excitations from the core level to specific unoccupied molecular orbitals characteristic of the internal bonding of the molecule. From the response of the strength of these features to changes in the polarization of the X-ray beam, the orientation of the molecule with respect to the surface can be determined. Shifts in position of the features can also be related to internal bond length changes to an accuracy of about 0.05 A. NEXAFS features are also sensitive to such things as oxidation states and chemical coordination. They can be used, for example, to identify different chemical groupings at polymer surfaces, or to identify reaction products at metal-semiconductor interfaces. XPD is an extension of XPS (Chapter 5). In XPS X rays of a fued wavelength, hv, eject core-level electrons from atoms in the sample. The kinetic energy, KE of these photoelectrons is measured, thereby allowing a determination of the corelevel BEs (hv-KE), which provides an atomic identification and chemical state information in some cases. In XPD the angular distributions of these atom specific photoelectrons are measured for materials possessing long range order (usually single crystal s&es, or for adsorption or reaction at these surfaces). The angular distributions are generated by diffraction of the photoelectron emitted from the target atom by its neighboring atoms. At the photoelectron energies involved the electron-atom scattering events peak in the forward direction, leading to the simple result that intensity maxima occur along atomic rows. Direct crystallographic infor196

ELECTRON/X-RAY DIFFRACTION ...

Chapter 4

mation about the near surroundings of the target atom is thus obtained. By comparing detailed calculations of the scattering processes for assumed models to the data, atom spacings can be obtained to an accuracy of 0.05 8.The probing depth is the same as for XPS; a few atomic layers. Strain, alloying, island formation, or interdiffusion during epitaxial growth is observable because the angular distributions are sensitive to atoms moving to “off-site” positions. XPD has been used as a simple diagnostic tool for distinguishing growth modes in well-ordered systems. Auger Electron Diffraction, AED, is an exact analogy to XPD, providing basically the same information. Instead of measuring the angular distribution of the ejected photoelectrons one uses the Auger electrons (Chapter 5).

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4.1

XRD X-Ray Diffraction M I C H A E L F. TONEY

Contents Introduction Basic Principles Experimental Methods for Thin Film Characterization by XRD Examples of XRD Characterization of Thin Films Conclusions

Introduction X-ray Diffraction (XRD) is a powerful technique used to uniquely identify the crystalline phases present in materials and to measure the structural properties (strain state, grain size, epitaxy, phase composition,preferred orientation, and defect structure) of these phases. XRD is also used to determine the thickness of thin films and multilayers, and atomic arrangements in amorphous materials (including polymers) and at interfgces. XRD offers unparalleled accuracy in the measurement of atomic spacings and is the technique of choice for determining strain states in thin films. XRD is noncontact and nondestructive, which makes it ideal for in situ studies. The intensities measured with XRD can provide quantitative, accurate information on the atomic arrangements at interfgces(e.g., in multiilayers).Materials composed of any element can be successfully studied with XRD, but XRD is most sensitive to high-2 elements, since the diffracted intensity from these is much larger than from low-2 elements. As a consequence, the sensitivity of XRD depends on the material of interest. With lab-based equipment, surfice sensitivities down to a thickness of -50 %i are achievable, but synchrotron radiation (because of its higher intensity) 198


Chapter 4

the characterization of much thinner films, and for many materials, monatomic layers can be analyzed. While the structure as a function of depth is not normally measured in XRD, this is possible using specialized methods. Alternatives to XRD include transmission electton microscopy (TEA4) and diffraction, Low-Energy and Reflection High-Energy Electron Diffiaccion (LEED and RHEED), extended X-ray Absorption Fine Structure (FXAFS), and neutron difhaction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivityin neutron diffraction is quite different fiom XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron dif€i-action is a good alternative for low-2 materials and fbr materials where the magnetic structure is of interest. While XRD is nondestructive and can be used in most environments, TEM and electron d k c t i o n are destructive techniques (due to specimen preparation methods) and require high vacuum. One of the disadvantages of XRD, compared to elecrron diffraction, is the low intensity of diffracted X rays, particltlarly for low-Z materials. Typical intensities for electron diffraction are 10’ times larger than for XRD. Because of small diffracted intensities, thin-film XRD generally requires large specimens (-0.5 cm)and the information acquired is an average over a large area. Usually, XRD does not provide spatial resolution, but for special applications, resolution of greater than 10 pm can be obtained with a microfocus source and a suitably thick film l p ) .The use of intense synchrotronX-ray radiation mitigates these two disadvantages somewhac however, the XRD analysis of thin filmswith synchrotron radiation is nor routine (becauseofthe limited accessibilityof synchrotron sources). Thin-film XRD is important in many technological applications, because of its abilities to accurately determine strains and to uniquely identifj, the presence and composition of phases. In semiconductor and optical materials applications, XRD is used to measure the strain stare, orientation, and defects in epitaxial thin films, which affect the film’s electronic and optical properties. For magnetic thin films, it is used to identifv phases and to determine preferred orientations, since these can determine magnetic properties. In metallurgical applications, it is used to determine strains in s d c e layers and thin frlms, which influence their mechanical properties. For packaging materials, XRD can be used to investigate diffusion and phase formation at interfixes all-

+

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Basic Principles

Thii section briefly discusses the fundamental principles of XRD; the reader is referred KO the works by Warren, Cullity, and Schwartz and Cohen for more detail.’-3 Figure 1 shows the basic features of an XRD experiment, where the diffraction angle 2Bis the angle between the incident and diffractedX rays. In a typical 4.1

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199

incident X-rays

-- I---F

Figure 1

diffracted

Basic features of a typical XRD experiment.

experiment, the diffracted intensity is measured as a function of 28and the orientation of the specimen, which yields the diffraction pattern. The X-ray wavelength A is typically 0.7-2 A, which corresponds to X-ray energies (E = 12.4 keV/A) of 6 17 keV. The Directions of Diffracted X Rays

Before considering the conditions for XRD, we will briefly review some important properties of crystalline materials. Crystals consist of planes of atoms that are spaced a distance d apart (Figures 1 and 2), but can be resolved into many atomic planes, each with a different d-spacing. To distinguish between these, we introduce a coordinate system for the crystal whose unit vectors a, b, and c are the edges of the unit cell (Figure 2b). For the familiar cubic crystal, these form an orthogonal system. Any atomic plane can now be uniquely distinguished by its Miller indices. These are the three reciprocal intercepts of the plane with the a-, b-, and c-axes and are reduced to the smallest integers having the same ratio. Thus, an (hkl) plane intercepts the crystallographic axes at a/h, b/k, and 4;examples are shown in Figure 2. The d-spacing between (hkl) planes is denoted dhH, and for cubic crystals, it is 200


Chapter 4

d210,

a

b

(111) Figure 2

(012 )

Several atomic planes and their &spacings in a simple cubic (sc)crystal (a); and Miller indices of atomic planes in an sc crystal (b).As an example consider the (012)plane. This interceptsthe a-, b-, and c-axes at -, 1, and 112. respectively, and thus, h = 1/- = 0,k = 1/1= 1, and I = 1/(1/2)= 2.

aO

dhkl =

JTh2+k2+2

where is the lattice constant of the crystal (see Figure 2). When there is constructive interference from X rays scattered by the atomic planes in a crystal, a diffraction peak is observed. The condition for constructive interference from planes with spacing dhH is given by Bragg's law:

h = 2dhklsinehkl where 8hu is the angle benveen the atomic planes and the incident (and diffracted) X-ray beam (Figure 1). For diffraction to be observed, the detector must be positioned so the diffraction angle is 28hu, and the crystal must be oriented so that the normal to the diffracting plane is coplanar with the incident and diffracted 4.1

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X rays and so that the angle between the difiacting plane and the incident X rays is equal to the Bragg angle For a single crystal or epitaxial thin film, there is only one specimen orientation for each (hkl) plane where these diffraction conditions are satisfied. Thin films, on the other hand, can consist of many grains or crystallites (small crystalline regions) having a distribution of orientations. If this distribution is completely random, then diffraction occurs from any crystallite that happens to have the proper orientation to satisfy the diffraction conditions. The diffracted X rays , a emerge as cones about the incident beam with an opening angle of 2 8 ~creating “powder” diffraction pattern. Thin films are frequently in a class of materials intermediate between single crystals and powders and have fiber texture. That is, all the crystallites in the film have the same atomic planes parallel to the substrate surface, but are otherwise randomly distributed. Face-centered cubic (fcc) films often grow with (1 11) fiber texture: The (1 11) planes are parallel to the substrate plane and planes perpendicular to this-e.g., (220)-are necessarily perpendicular to the substrate but otherwise randomly distributed. The resulting diffraction pattern consists of rings about the film normal or (111) axis. Thin films may not have complete fiber texture, but may possess a preferred orientation, where most, but not all, of the crystallites have the same atomic planes parallel to the substrate. For example, while most of the fcc film might have (111) texture, a few of the crystallites might have their (200) planes parallel to the substrate. As we have seen, the orientation of crystallites in a thin film can vary from epitaxial (or single crystalline), to complete fiber texture, to preferred orientation (incomplete fiber texture), to randomly distributed (or powder). The degree of orientation not only influences the thin-film properties but also has important consequences on the method of measurement and on the difficulty of identifying the phases present in films having multiple phases. Intensities of Diffracted X Rays

Before considering diffracted intensities, we first must consider X-ray absorption, since this affects intensities. All materials absorb X rays. Thus, an X-ray beam is attenuated as it traverses matter. The transmitted intensity decays exponentially with the distance traveled through the specimen and the linear absorption coefficient p describes this decrease. The absorption length (1/e decay length) is 1 /p, and at A = 1.54 A, typical values are lmm, 66 p,and 4pm for carbon, silicon, and iron, respectively. Except near an absorption p increases with increasing atomic number and increasing wavelength. Neglecting unimportant geometric the integrated X-ray intensity diffracted from a thin film is

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Chapter 4

Here Fhu is the structure hctor for the (hkl) diffraction peak and is related to the atomic arrangements in the material. Specifically, FM is the Fourier transform of the positions of the atoms in one unit cell. Each atom is weighted by its form factor, which is equal to its atomic number 2 for small 28, but which decreases as 28 increases. Thus, XRD is more sensitive to high-2 materials, and for low-2 materials, neutron or electron diffraction may be more suitable. The factor e-2M(called the Debye-Waller factor) accounts for the reduction in intensity due to the disorder in the crystal, and the diffracting volume V depends on and on the film thickness. For epitaxial thin films and films with preferred orientations, the integrated intensity depends on the orientation of the specimen. DynamicalX-Ray Diffraction

In the concepts developed above, we have used the kinematic approximation, which is valid for weak diffraction intensities arising from “imperfect” crystals. For perfect crystals (available thanks to the semiconductor industry), the diffraction intensities are large, and this approximation becomes inadequate. Thus, the dynamical theory must be used. In perfect crystals the incident X rays undergo multiple reflections from atomic planes and the dynamical theory accounts for the interference between these reflections. The attenuation in the crystal is no longer given by absorption (e.g., p) but is determined by the way in which the multiple reflections interfere. When the diffraction conditions are satisfied, the diffracted intensity from perfect crystals is essentially the same as the incident intensity. The diffraction peak widths depend on 2& and Fm and are extremely small (less than +O.OOl-0.005’). Experimental Methods for Thin Film Characterization by XRD

Because the diffracting power of thin films is small, the instrumentation and techniques h r thin-film XRD are designed to maximize diffracted intensities and to minimize background. There are basically two classes of measurement techniques. The first, and oldest, uses photographic film; these methods provide fist, preliminary information and yield two-dimensional data. However, progress in computers and high-power X-ray generators has lead to the widespread use of difiactometers, where the diffracted X rays are detected with photon counters. Compared to photographic methods, counters provide more accurate, quantitative data and have superior signal-to-noise ratios. Furthermore, difiactometers are easily automated and provide better angular resolution. Recently, there has been increasing use of which use parallel detection to s c a n a range in 26. position-sensitive X-Ray Diffractometer Methods

The BraggBrentano geometry ’-4 is used widely for preferentially and randomly oriented polycrystalline films. In this geometry (Figure 3a), slits collimate the inci4.1

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203

/;;:mating

rotating

\

X-ray source

a detector

fixed specimen

A

,-A

X-ray sourc diffractometer

Figure 3

Bragg-Brentano dffractometer (a); and Seernann-Bohlin diffractorneter (b). The point F is either the focal point on an X-ray tube or the focal point of a focusing monochromator.

dent X rays, which impinge on the specimen at an angle 8. Afier passing through receiving slits, the diffracted X rays are detected. The specimen is rotated at onehalf the angular velocity of the detector. Since the incident and diffracted X rays make the same angle to the specimen surface, structural information is obtained only about (hkl) planes parallel to this surhce. When the receiving slits, the specimen, and the focal point F lie on a circle, the diffracted X rays are approximately focused on the receiving slits (parafocusing),which considerably improves the sensitivity. For the Seemann-Bohlin geometry (Figure 3b) the incident X rays impinge on a fmed specimen at a small angle y + 5-10" and the diffracted X rays are recorded by a detector that moves along the focusing circle." This method provides good sensitivity for thin films, due to parafocusing and the large diffracting volume, which results from ybeing small and the X-ray path length in the film being large (propor204


Chapter 4

tional to 1/shy). Because yis fixed, the angle between the incident X rays and the diffracting planes changes as the detector moves through 28. Because only planes with the correct orientation diffract X rays, this method is most usell for polycrystalline films having random or nearly random crystallite orientations. A Double-Crystal Diffractometer (DCD) is us& for characterizingnearly perfect, epitaxial thin films.& This geometry is similar to the Bragg-Brentano case, but the incident beam is first diffracted from a perfect single crystal (placed near F in Figure 3a). It is thus monochromatic and well collimated. This insures that the measured diffraction peak width of the specimen is narrow, thereby permitting high-resolution measurements. Typically, the detector is fured near 2%, the diffraction angle for the (hkl) planes of interest, and the receiving slits are open to accept a large range in 28. The intensity is recorded in a “rocking curve,” where the specimen rotates about e,. Asymmetric reflections, where the (hkl) planes are not parallel to the substrate can also be measured. Since the diffraction peak widths in DCD measurements are narrow, this method enables accurate determination of very small deviations in d-spacings due to strain. For ultrathin epitaxial fdms (less than 100A), GrazingIncidenceX-ray Diffraction (GIXD) is the preferred method6 and has been used to characterize monolayer films. Here the incidence angle is small (-0.5’) and theX rays penetrate only 100200 8,into the specimen (see below). The exit angle of the diffracted X rays is also small and structural information is obtained about (hkl) planes perpendicular to the specimen surface. Thus, GMD complements those methods where structural information is obtained about planes parallel to the surface (e.g., Bragg-Brentano and DCD).

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Photographic Methods

Photographic methods24 of characterizing polycrystalline thin films are used to acquire preliminary data and to determine orientational relationships. Guinier and Read cameras are common, although other methods are also used. In a Guinier camera the geometry is the same as for a Seemann-Bohlin difhactometer with a focusing monochromator, except that the “detector”is now a cylinder of film. The geometry of a Read camera is similar to the Bragg-Brentano difiactometer, but the incidence angle is fixed (-5-10’) and the cylindrical film spans a wide range in 28. X-ray topography is a photographic method used to image defects in nearly perfect single cry~tals.~.The topograph is a map of the diffracted intensity across the specimen. There are several topographic techniques useful for thin films; 3, we only note a few of their capabilitieshere. The Berg-Barren and section methods are simple and give good surface sensitivity. The Lang method (scanning reflection) is more complicated, but large areas can be imaged with good s& sensitivity and spatial resolution. These methods all use a single perfect crystal-the specimen. In the double-crystal method, a reference crystal is used also to produce a monochromatic, collimated incident beam. Although more complicated, this method pro4.1

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205

vides maximum sensitivity to defects and can provide good surface sensitivity and spatial resolution. Examples of XRD Characterizationof Thin Films Phase Identification

One of the most important uses of thin-film XRD is phase identification.Although other techniques (e.g., RBS, X P S , and XRF) yield film stoichiometries, XRD provides positive phase identification. This identification is done by comparing the measured d-spacings in the diffraction pattern and, to a lesser extent, their integrated intensities with known standards in the JCPDS Powder Diffraction File (Joint Committee on Powder Diffraction Standards, Swathmore, Pennsylvania, 1986). However, thin films often have a preferred orientation, and this can cause the measured intensities to disagree with the JCPDS file, which are for random orientations. For films containing several phases, the proportion of each phase can be determined from the integrated intensities in the diffraction pattern. If the phases in the film have random orientation or almost complete fiber texture, this determination is simple.2d However, if there is some preferred orientation (incomplete fiber texture), the determination of phase proportions may require integrated intensities at many specimen orientations, which is time consuming. Furthermore, for multiphase specimens, preferred orientation can make positive phase identification difficult, since the integrated intensities may not be useful for phase identification. (For example, peaks that are strong in powder patterns may be weak or completely absent in a specimen with preferred orientation). This difficulty can be particularly acute if data are available only for one specimen orientation (i.e., the Bragg-Brentano geometry) or if the phases produce many diffraction peaks. Other excellent methods of phase identification include TEM and electron diffraction. These may be more useful for low-Z materials, ultrathin films, and for characterizing small areas, including individual grains. For multiphase films with incomplete texture, these methods and XRD are complementary, since in commonly used geometries, they probe atomic planes perpendicular and parallel to the thin film surfice, respectively. Figure 4 shows an example where XRD is used to unambiguously identify the phases in three high-Tc superconducting thin films.8 Since the films have nearly complete fiber texture (see below), the identification was simple and was done by comparison to the diffraction patterns from bulk materials. Furthermore, from comparison to standards, the presence of a small amount of CuO is apparent in one film (Figure 4a). We also conclude that the film in Figure 4b consists of approximately equal mixtures of Tl2CaBa~Cu~0, and T12Ca2Ba2Cu30y,since it can be reproduced by an approximately equal combination of the patterns in Figures 4a and 4c. Agam, because of the strong fiber texture, this determination is straightfor206


Chapter 4

30

20

10

40

50

0

28 (degrees1

Figure 4

Diffraction patterns (Bragg-Brentano geometry) of three superconducting thin films (-2ym thick) annealed for different times! The temperatures for 0 resistanceand for the onset of superconductivityare noted.

ward. Figure 5 shows the XRD pattern from a bilayer film of Co70Pt12Cr18/Cr used for magnetic recordingg Here the phase of the CoPtCr magnetic media was shown to be hexagonal close packed (hcp) by comparing the measured peaks (including some not shown) with those expected for an hcp solid solution Co70Pt12Cr18m

Determination of Strain and Crystallite Size

Diffraction peak positions, and therefore, atomic spacings are accurately measured with XRD, which makes it 'the best method for characterizing homogeneous and inhomogeneous 6, lo Homogeneous or uniform elastic strain shifts the diffraction peak positions, and if do, hH is the unstrained d-spacing, (du-do, hk) / do, u is the component of elastic strain in the (hkl) direction. Figure 6 shows5 a rocking curve of 2500-A& & a 0 ~ 2 h film on G a b and illustrates the superb resolution possible with XRD. From the shift in peak positions, one can calculate the difference in d-spacings between the thin film and substrate (in the (100) direction). Although this is oniy 0.231%, the diffraction peaks from &e film and substrate are easily distinguished. Furthermore, the variation in the d-spacing of the fdm near the film-substrate interface is determined from modeling the data.5 Inhomogeneous strains vary from crystallite to crystallite or within a single crystallite and this causes a broadening of the diffraction peaks that increaseswith sin 8. Peak broadening is also caused by the finite size of crystallites, but here the broad4.1

XRD

207

ening is independent of sin e. When both crystallitesize and inhomogeneous suain contribute to the peak width, these can be separately determined by car& analysis of peak shapes for several diffraction orders (e.g., ( l l l ) , and (222)).l" Furthermore, the diffraction peak shape can provide information on other types of imperfections (i.e., the presence, extent, and type of stacking faults).'* If there is no inhomogeneous strain, the crystallite size L is estimated from the peak width 428 with the Scherrer formula:

L-

h

(4)

(A28) cose

-

Using this and the data in Figure 5, one estimates L 180 A for the CoPtCr film.' Grain or crystallite size are also determined with TEM through direct imaging. Since this method is a local probe, it can provide more detailed information on imperfections thanXRD. Although strain gauges can measure homogeneous strain, there is no good alternative to XRD for strain measurements. Determination of Preferred Orientation

For polycrystalline films, the amount of prefkrred orientation can be estimated by comparing the integrated intensities (after correction for geometric factors) to the JCPDS file or the expression for integrated intensity (Equation (3)).If the film has (hkl) fiber texture, then in the Bragg-Brentano geometry, the (hkl) d&ction peak will have a larger relative intensity than expected. Figure 4 shows an example of nearly complete (001) fiber texture in high-Tc thin films, since only the (001)peaks the (002) peak is observed but is are observed.8 For the CoPtCr media (Figure 9, weak compared to the (100) and (101) peaks. Thus, the preferred orientation for the (001) axis is to lie in the plane.' Although this fdm possess incomplete fiber texture, phase identification is straightforward, since the reasonable phase possibilities (hcp, fcc, and bcc) are easily distinguished. To obtain a more quantitative determination of preferred orientation, the intensity of an (hkl) peak is measured at different specimen orientations, for example with a pole-figure goniometer.2 Photographic methods are particularly usell fbr this, since they provide twodimensional information, but are less quantitative. For epitaxial films, GIXD and DCD are used to determine thin f h orientation. Preferred orientation is also measured with TEM, although less quantitatively. Film ThicknessDetermination

The film thickness of epitaxial and highly textured thin films can be measured with XRD.43 Close to the usual or primary diffraction peaks there are secondary or subsidiary maxima in the difFracted intensity (see Figure 6), which are due to the finite film thickness.'. The film thickness is inversely proportional to the spacing between these maxima and is easily calculated. X-ray reflectivity is another accurate method for measuring a film's thickness. 208


Chapter 4

40

Figure 5

45 28 (Degrees)

50

Bragg-Brentano d e a d i o n pattern for magnetic media used in a demonstration of 1Gb/in2 magnetic recording! The lines show a deconvolut-onof the data into individual diffractionpeaks, which are identified.

Depth-Dependent Information

In most thin-film XRD analyses, depth-dependent structural information is not obtained, but recently such measurements have been performed using a grazing incidence geometry." Since the refractive index for X rays'. is less than 1, X rays experience total external reflection at incidence angles less than the critical angle for total reflection (q). By varying the incidence angle near q, the penetration depth of the incident X rays is varied from +50 A up to several pm. Since the diffracted X rays originate fiom different depths, the depth-dependent structure of the specimen may be obtained from diffraction patterns taken at different incidence angles. Depth-dependent structure can also be obtained from TEM, although less quantitatively. Interdiffusion of bilayered thin films also can be measured with XRD.'', l3 The diffraction pattern initially consists of two peaks from the pure layers and after annealing, the diffracted intensity between these peaks grows because of interdiffusion of the layers. An analysis of this intensity yields the concentration profile, which enables a calculation of diffusion coefficients, and diffusion coefficients -10'2-10-'5 cm2/s are readily measured.12 With the use of multilayered specimens, extremely small diffusion coefficients (+1 0-23cm2/s) can be measured with XRD.l2,l3 Alternative methods of measuring concentration profiles and diffusion coefficients indude depth profiling (which suffers from artificts), RBS (which can not resolve adjacent elements in the periodic table), and radiotracer methods (which are difficult). For XRD (except for multilayered specimens), there must be a unique relationship between composition and the d-spacings in the initial films and any solid solutions or compounds that form; this permits calculation of the compo4.1

XRD

209

100.0

10.0

z L

: 1.0

0)

c .-c 0

0)

u-

0

IT:

0.1

0.01 -0.2

-0.1

0.0

0.1

A 8 (degrees) Figure 6

DCD rocking curves-meawred (dashed) and calculated (solidkof the (4001 diffraction peak from AIo.,Ga0,,As on GaAs(100)FThe arrows mark the subsidiary maxima.

sition from the diffraction peak positions. O n the other hand, the nondestructive nature of XRD makes it a very powerful technique for the measurement of concentration profiles. X-Ray Topography

’

X-ray topography3’ is used to image defects in nearly perfect epitaxial films and the surface layers of substrates. Typical defects include dislocations, precipitates, fault planes, and local blisters and cracks, and these may be present in the thin film or substrate, or may have been induced into the substrate by the presence of the film. An X-ray topograph is a image of the diffracted intensity across the specimen. Contrast is produced by local variation in the intensity because of changes in the diffraction conditions and is most often due to the strain fields associated with defects. A topograph must be interpreted with the dynamical theory. Although the lateral resolution of X-ray topography (- 1 pm) is about 1000 times poorer than TEM, much 210


Chapter 4

smaller strains (-lo4) and larger areas (+1-10 cm) can be imaged. Using conventional methods, layers as thin as 1 pm are imaged, but by using grazing incidence angles, the minimum thickness that can be imaged is about 0.1 pm. If the diffraction peaks from a thin film and substrate occur at sufficientlydifferent 20, an image of the peak from the thin-film maps only the defecrs in the film. Characterization of Multilayered Films

Recently, there has been remarkable progress in the controlled synthesis of multilayered materialsy6Il 3 which have a repeating modulation in chemical composition. The atomic structure of multilayers, which influences their properties, is readily studied with XRD. Most XRD investigationshave concentrated on structure along the growth direction and this involves measuring atomic planes parallel to the surface, often with Brag-Brentano or DCD geometries. The wavelength of the modulation in composition (modulation wavelength) and the average strain are determined from the diffraction peak positions; phase identification and measurements of the crystallite size and the preferred orientation are performed as described earlier. A 111 characterization and understanding of these materials requires accurate knowledge of the interhcial roughness and the composition and position of the atomic planes in a modulation wavelength. This is difficult, since it requires careful analysis of the peak shapes and a complete set of integrated intensities. A determination of the structure in the surface plane can also be important and is performed with GIXD; however, this often is not done. Alternative characterization techniques include neutron diffraction and reflectivity, TEM, EXAFS, LEED, and RHEED, although none of these provide the quantitative detail available with XRD. RHEED, however, is used to monitor multilayer growth in situ. Characterization of Amorphous Materials

The diffraction pattern from amorphous materials (including many polymers) is devoid of the sharp peaks characteristic of crystals and consists of broad features or halos.'! Quantitative analysis of XRD data from amorphous materials is complicated but provides important information on the local atomic structure (shortrange order), including the bond lengths, the number of neighbors, and the extent of atomic correlations. Since the diffraction from amorphous materials is weak, thick specimens or synchrotron radiation is necessary, particularly for low-Z materials. Many polymers are amorphous or semicrystalline, and for polymeric materials, XRD is used to probe the structure, morphology, and degree of ~rystallinity.'~ TEM is a widely used alternative for amorphous materials, but is less quantitative and can damage polymers. EXAFS is also widely used and complements XRD but cannot be used for materials composed of only low-Z elements. 4.1

XRD

211

Conclusions

XRD is an excellent, nondestructive method for identifjhg phases and characterizing the structural properties of thin films and multilayers. It is inexpensive and easy to implement. The future will see more use of GIXD and depth dependent measurements, since these provide important information and can be carried out on lab-based equipment (rather than requiring synchrotron radiation). Position sensitive detectors will continue to replace counters and photographic film. Multilayered materials will become more important in the future, and therefore, their structural characterization using XRD will grow in importance. The use of synchrotron radiation as an analytical tool for thin film characterization will also increase. The unique characteristicsof this radiation enable formerly difficult labbased experiments to be done simply (e.g., GIXD and depth-dependent structural measurements) and permit experiments that are otherwise impossible. These indude thin-film energy dispersive XRD, where the incident beam is polychromatic and the diffraction from many atomic planes is obtained simultaneously,and anomalous diffraction, where the abrupt change in the diffracting strength of an element near a particular X-ray energy (an absorption edge) is used to differentiate elements.*. Related Articles in the Encyclopedia

TEM, EXAFS, M E E D , LEED, Neutron Diffraction References 1

B. E. Warren. X-Ray Difhaction. Addison-Wesley, Reading, 1969. A dassic text that is complete and thorough, although somewhat dated.

2

B. D. Cullity. Elements of X-Ray Diffraction. Addison-Wesley, Reading, 1978. Another classic text, which is simpler than Warren and emphasizes metallurgy and materials science. A good introduction.

3

L. H. Schwartz and J. B. Cohen. Diffraction from Materials. SpringerVerlag, Berlin, 1987. A recent text that includes X-ray, neutron, and electron diffraction, but emphasizesXRD in materials science. A good introduction and highly recommended.

4

A. Segmuller and M. Murakami. Characterization of Thin Films by XRay Diffraction. In: Thin Films from Free Atoms and Particles. (KJ. Klabunde, ed.) Academic Press, Orlando, 1985, p.325 A recent brief review article with many references. I? S. Speriosu, M. A. Nicolet, J. L. Tandon, andY. C. M. Yeh. Interfacial Strain in AlGaAs Layers on GaAs. J. Appl. Phys. 57, 1377, 1985.

5

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A. Segmuller, I. C. Noyan, V.S. Speriosu. X-Ray Diffraction Srudies of Thin Films and Multilayer Structures. hog. Cryst. Growth and Charact. 18,21, 1989. 7 D. K. Bowen. X-Ray Topography of Surface Layers and Thin Films. In: Advances in X-ray Analysis. (C.S. Barren, J. V Gilfrich, R. Jenkins, and I? K. Predecki, eds.) Plenum, New York, 1990, vol. 33, p. 13. 8 W. Y. Lee, V.Y. Lee, J. Salem,T. C. Huang, R Savoy, D. C. Bullock and S. S. F? Parkin. SupercanductingTlCaBaCuO Thin Films with Zero Resistance at Temperatures of up to 120K. Appl. Phys. Lett. 53,329, 1988. 9 T. Y. Yogi, C. Tsang,T. A. Nguyen, K. Ju, G. L. Gorman, and G. Castillo. Longitudinal Magnetic Media for 1Gb / Sq. In. Areal Densiv. IEEE Trans. Magn. MAG-26,2271, 1990. i o A. Segrnuller and M. Murakami. X-Ray Diffraction Analysis of Strains and Stresses in Thin Films. In: Analytical Techniquesfor Thin Films. (KN. Tu and R Rosenberg, eds.) Academic, San Diego, 1988, p.143. 11 M. F. Toney and S. Brennan. Structural Depth Profiling of Iron Oxide Thin Films using Grazing IncidenceAsymmetric Bragg X-ray Diffraction. J. Appl. Phys. 65,4763, 1989. 12 M. Murakami, A. Segmuller, K. N. Tu. X-Ray Difhacrion Analysis of Difh i o n in Thin Films. In: Analytical Techniques fbr Thin Films. (K. N. Tu and R. Rosenberg, eds.) Academic Press, San Diego, 1988, p.201. i s l? Dhez and C. Weisbuch. Physics, Fabrication, and Applications of Multilayered Structures. Plenum, New York, 1988. 14 M. Kakudo and N. Kasai. X-ray Difkmion by Polymers. Elsevier,Tokyo, 1972. 6

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213

4.2

EXAFS ExtendedX-Ray Absorption Fine Structure MARK R. A N T O N I O

Contents

Introduction Experimental Aspects Basic Principles Data Analysis Capabilities and Limitations Applications Conclusions

Introduction

The discovery of the phenomenon that is now known as extended X-ray absorption fine structure (EXAFS) was made in the 1920s, however, it wasn't until the 1970s that fwo developments set the foundation for the theory and practice of EXAFS measurements.' The first was the demonstration of mathematical algorithms for the analysis of EXAFS data. The second was the advent of intense synchrotron radiation of X-ray wavelengths that immensely facilitated the acquisition of these data. During the past two decades, the use of EXAFS has become firmly established as a practical and powerful analytical capability for structure determination.24 214


Chapter 4

EXAFS is a nondestructive, element-specific spectroscopictechnique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfkes, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing dement in terms of the numbers, types, and interatomic distances of neighboring atoms. EXAFS is part of the field of X-ray absorption spectroscopy &AS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternativelycalled the near-edge X-ray absorption fine structure, NEXAFS). The combination ofXANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article followingthii one fbr a discussion of SEXAFS and NEXAFS. ExperimentalAspects

An EXAFS experiment involves the measurement of the X-ray photoabsorption of a selected element as a hnction of energy above its core-shell electron binding energy. The most direct measurement of EXAFS is the transmission method, wherein the sample is placed in the X-ray beam and the incident and transmitted X-ray intensities, 10 and 4, respectively, are recorded (see Figure 1). The measurement of Io and I,is accomplished with two ion chamber proportional counters that are gas filled (typically with nitrogen and argon) to provide about 10-20% absorption of Io and 80-30% absorption of I,. As shown in Figure 1, it is useful to have a third ion chamber for simultaneous measurements of a reference material (e.g., a thin metal foil) to maintain accurate energy calibration throughout the course of experiment. For successful transmission measurements, the ideal sample thickness x is one absorption length, i.e., x = l/[(p/p)p]; here p/p is the total mass absorption coefficient and p is the density. Transmission EXAFS data for samples with larger absorption lengths can be seriously distorted and are not suitable for analysis? Transmission EXAFS data are displayed in the form 1n(Io/1. versus incident X-ray energy, as shown in Figure 2. A wide selection of metal reference foils and powder films of ideal thickness for tranmission EXAFS is available from The EXAFS Materials Company, Danville, CA,USA. The transmission method is well-suited for in situ measurements of materials under industrially relevant conditions of extreme temperature and controlled atmosphere. Specially designed reactors for catalysis experiments and easy4.2

EXAFS

215

MONOCHROMATOR

I

Figure 1

Schematic view of a typical EXAFS experiment at a synchrotron radiation facility. Note that it is possible to record transmission and fluorescence EXAFS simultaneouslywith reference EXAFS.

to-use detectors are commercially available from The EXAFS Company, Seattle,

WAYUSA. In addition to transmission, EXAFS data can be recorded through the detection of 1

X-ray fluorescence

z Electron yield 3

Ion yield

4

Optical luminescence

5

Photoconductivity

Photoacoustic signals. The last three detection schemes apply only under very special circumstances.s Transmission EXAFS is strictly a probe of bulk structure, i.e., more than about a thousand monolayers. The electron- and ion-yield detection methods, which are used in reflection rather than transmission schemes, provide surface sensitivity, 11,000 A, and are inherently insensitive to bulk structure. X-ray fluorescence EXAFS has the widest range of sensitivity-from monolayer to bulk levels. The combination of electron or ion yield and transmission EXAFS measurements can provide structural information about the X-ray absorbing element at the surface and in the bulk, respectively, of a sample. Without exception, the highest quality FXAFS data are acquired at synchrotron radiation facilities. There are 20 operational kcilities throughout the world.'' Each has unique instrumentation: The interested user is encouraged to contact the ficil6

+

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Chapter 4

h

J

7 CI

B

a

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figure 2

1

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Molybdenum K-edge X-ray absorption spectrum, ln(&4) versus X-ray energy (ev), for molybdenum metal foil (25-pn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-ray Absorprion Fine structure 6.S. Hasain, ed.) Ellis Horwood, New York, 1991.

ity for detailed information, such as is available in Gmur.l1 In general, “hard”X-ray beam lines (approximately 2 2,000 ev) employ flat-crystal monochromators to

scan the X-ray energy over the region of interest, whereas “soft”X-ray beam lines (approximately I 1,000 ev) employ grating-type monochromators for the same purpose. The monochromatization of X rays with energies between approximately 1,000 and 2,000 eV is a difficult problem-neither crystal nor grating monochromators work particularlywell. Basic Principles

Both inner-shell (K and L) and outer-shell (M, N, etc.) electrons can be excited by the absorption of X rays and by the inelastic scattering of electrons. In either instance, at an electron binding energy characteristic of an element in a sample, 4.2

EXAFS

217

Figure 3

Schematic illustration of the EXAFS phenomenon: (A) outgoing photaelee tron (solid curve) from X-ray absorbing atom; (8)destructive interference at the absorbing atom between outgoing (solid curve) and backscattered (dashed curve) photoelectron from neighboring atom; (C) constructive inter ference at the absorbing atom between outgoing (solid curve) and backscattered (dashed curve) photoelectronfrom neighboring atom. Adapted from T. M. Hayes and J. B. Boyce. Solidstate Phys 37,173,1982.

absorption occurs and a steeply rising absorption edge is observed. For example, molybdenum exhibits an X-ray absorption edge at 20,000 eV, which is the 1s electron binding energy (K edge) ,see Figure 2. The pre-edge and edge features are collectively referred to as XANES or NEXAFS, depending upon the application. These data are valuable for probing the site symmetry and valence of the X-ray absorbing element, but will not be discussed further here. For X-ray energies greater than the binding energy, the absorprion process leads to the excitation of the core electron to the ionization continuum. The resulting photoelectron wave propagates from the X-ray absorbing atom and is scattered by the neighboring atoms, as illustrated in Figure 3. The EXAFS spectrum results from the constructive and destructive interference between the outgoing and incoming photoelectron waves at the absorbing atom. The interference gives rise to the modulatory structure (i.e., peaks and valleys) of the X-ray absorption versus incident X-ray energy, as in Figure 2. This process also makes EXAFS unique-the absorbing atom acts as both the source and detector of the interference that is the EXAFS phenomenon. EXAFS is a probe of the structural distribution, e.g., interatomic distances, numbers of neighboring atoms (the so-called coordination number), and degree of disorder-and identity of atoms in the immediate vicinity (-5 A) of the X-ray absorbing atom. A simplified schematic representation of several descriptive features of EXAFS is presented in Figure 4.The frequency of EXAFS oscillations is related to the distance between the X-ray absorbing atom (filled circles) and the backscattering atoms (open circles). For large inreratomic distances (Rl > RZ), the EXAFS has shorter periods (higher hequencies) than for small distances; see curves 218


...

Chapter 4

~~

~-

X-RAY PHOTON ENERGY

Figure 4

Descriptive aspects of EXAFS: Curves A 4 are discussed in the text. Adapted from J. Stohr. In: Emission and Scatering Techniques: Studies of Inorganic Molecules, Solids, and Surfsces. (P. Day, ed.) Kluwer, Norwell, MA, 1981.

A and Byrespectively, in Figure 4. The periodicity is also related to the identity of the absorbing and backscattering elements. Each has unique phase shihs.'* EXAFS has an energy-dependent amplitude that is just a few % ofthe total X-ray absorption. This amplitude is related to the number, type, and arrangement of backscattering atoms around the absorbing atom. As illustrated in Figure 4 (curve C), the EXAFS amplitude for backscattering by six neighboring atoms at a distance R is greater than that for backscattering by two of the same atoms at the same distance. The amplitude also provides information about the identity of the 4.2

EXAFS

219

backscatteringelement-each has a unique scattering function12-and the number of different atomic spheres about the X-ray absorbing element. As shown in Figure 4, the EXAFS for an atom with one sphere of neighbors at a single distance exhibits a smooth sinusoids decay (see curvesA X ) , whereas that for an atom with two (or more) spheres of neighbors at &%rent distances exhibits beat nodes due to superposed EXAFS signals of different frequencies (curve D). The EXAFS amplitude is also related to the Debye-Wder factor, which is a measure of the degree of disorder of the backscattering atoms caused by dynamic (i.e., thermal-vibrational properties) and static (i.e., inequivalence of bond lengths) e k t s . Separation of these two effects from the total Debye-Waller factor requires temperature-dependentEXAFS measurements. In practice, EXAFS amplitudes are larger at low temperatures than at high ones due to the reduction of atomic motion with decreasing temperature. Furthermore, the amplitude for six backscattering atoms arranged symmetrically about an absorber at some average distance is larger than that fbr the same number of backscattering atoms arranged randomly about an absorber at the same average distance. Static disorder about the absorbing atom causes amplitude reduction. Finally, as illustrated in Figure 4 (curve E), there is no EXAFS for an absorbing element with no near neighbors, such as for a noble gas. Data Analysis

Because EXAFS is superposed on a smooth background absorption po it is necess a r y to extract the modulatory structure p from the background, which is approximated through least-squares curve fitting of the primary experimental data with versus Ein Figure 2).', l2 The EXAFS specpolynomial functions (i.e., ln(I,/lf) trum x is obtained as x = [p%]/h. Here x, p, and po are functions of the photoelectron wave vector k (A-'), where R = [0.263(E-&)]'; & is the experimental energy threshold chosen to define the energy origin of the EXAFS spectrum in k-space. That is, k = 0 when the incident X-ray energy E equals &, and the photoelectron has no kinetic energy. EXAFS data are multiplied by k" (n= 1,2, or 3) to compensate for amplitude attenuation as a function of k, and are normalized to the magnitude of the edge jump. Normalized, background-subtracted EXAFS data, k%(R) versus k (such as illustrated in Figure 5), are typically Fourier transformed without phase shift correction. Fourier transforms are an important aspect of data analysis because they relate the EXAFS function R?(k) of the photodemon wavevector k to its of distance r'(&. Hence, the Fourier trandorm complementary function provides a simple physical picture, a pseudoradial distribution function, of the environment about the X-ray absorbing element. The contributions of different coordination spheres of neighbors around the absorber appear as peaks in the Fourier d o r m . The Fourier transform peaks are always shifted from the true distances t to shorter ones r' due to the &t of a phase shift, which amounts to +0.20.5 A, depending upon the absorbing and backscatteringatom phase functions.

a-')

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Chapter 4

21

-

14

-

7-

h

3

W

mx 3

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-14-

-21

-28

-

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4

6

8

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10

I

12

I

14

I

16

I

18

I

20

k in Inverse Angstroms Figure 5

Background-subtracted, normalized, and kJ-weighted Mo K-edge EXAFS, Px(kl versus k (Am'], for molybdenum metal foil obtained from the primary experimentaldata of Figure 2 with = 20,025 eV.

The Fourier transform of the EXAFS of Figure 5 is shown in Figure 6 as the solid curve: It has two large peaks at 2.38 and 2.78 A as well as two small ones at 4.04 and 4.77 A. In this example, each peak is due to Mo-Mo backscattering. The peak positions are in excellent correspondence with the crystallographically determined radial distribution for molybdenum metal foil (bcc)-with Mo-Mo interatomic distances of 2.725,3.147,4.450, and 5.218 A, respectively. The Fourier transform peaks are phase shifted by -0.39 A from the true distances. To extract structural parameters (e.g. interatomic distances, Debye-Waller factors, and the number of neighboring atoms) with greater accuracy than is possible from the Fourier transform data alone, nonlinear least-squares minimization techniques are applied to fit the EXAFS or Fourier transform data with a semiempirical, phenomenological model of short-range, single ~cattering.~. l2 Fourier-filtered EXAFS data are well suited for the iterative refinement procedure. High-frequency noise and residual background apparent in the experimental data are effectively removed by Fourier filtering methods. These involve the isolation of the peaks of interest from the total Fourier transform with a filter function, as illustrated by the dashed curve in Figure 6. The product of the smooth frlter with the real and imagi4.2

EXAFS

22 1

5

000 800 -

000

\

\

\ \ \ \

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IIII

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700

\\

\

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500 I

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I' I I I

300 200

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2

3

4

5

6

7

8

r' in Angstroms Figure6

versus r' (A, without phase-shift Fourier transform W i d curve), @&') correction), of the Mo K-edge EXAFS of Figure 5 for molybdenum metal foil. The-Fourier filtering window (dashed curve) is applied over the region -1.54.0 A to isolate the two nearest Mo-Mo peaks.

nary parts of the Fourier transform on the selected distance range is then Fourier inverse-transformed back to wavevector space to provide Fourier-filtered EXAFS, as illustrated by the solid curve of Figure 7. For curve fitting, phase shifts and backscattering amplitudes are fmed during the least-squares cycles. These can be obtained readily from theoretical or, alternatively, empirical tabulations.l2 The best fit (dashed curve) to the Fourier-filtered EXAFS data (solid curve) of the first two coordination spheres of molybdenum metal is shown in Figure 7.

Capabilitiesand Limitations

The classical approach for determining the structures of crystalline materials is through diffraction methods, i.e., X-ray, neutron-beam, and electron-beam techniques. Diffraction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 222


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Figure7

Fourier-filtered Mo Ksdge EXAFS, PX(k) versus k (Am1)(solid curve), for molybdenum metal foil obtained from the filtering region of Figure 6. This data is provided for comparison with the primary experimental EXAFS of Figure 5. The two-term Mo-Mo best fit to the filtered data with theoretical EXAFS amplitude and phase functions is shown as the dashed curve.

Because diffraction methods lack the element specificity of EXAFS and because EXAFS lacks the power of molecular-crystal structure solution of diffraction, these two techniques provide complementary information. On the one hand, diffraction is sensitive to the stereochemical short- and long-range order of atoms in specific sites averaged over the different atoms occupying those sites. O n the other hand, EXAFS is sensitive to the radial short-range order of atoms about a specific element averaged over its different sites. Under favorable circumstances, stereochemical details (Le., bond angles) may be determined from the analysis of EXAFS for both oriented and unoriented samples.l2 Furthermore, FXAFS is applicable to solutions and gases, whereas diffraction is not. One drawback of EXAFS concerns the investigation of samples wherein the absorbing element is in multiple sites or multiple phases. In either case, the results obtained are for an average environment about all of the X-ray absorbing atoms due to the element-specific site averaging of structural information. Although not common, site-selective EXAFS is po~sible.~ 4.2

EXAFS

223

Unlike traditional surfice science techniques (e.g., X P S , A E S , and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one’s own laboratory than at a synchrotron radiation ficility, which is therefore a significant drawback to the EXAFS technique. These facilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. Although not difficult, the acquisition of EXAFS is subject to many sources of error, including those caused by poorly or improperly prepared specimens, detector nonlinearities, monochromator artifacts, energy calibration changes, inadequate signal-to-noise levels, X-ray beam induced damage, et^.^ Furthermore, the analysis of EXAFS can be a notoriously subjective process: an accurate structure solution requires the generous use of model compounds with known structure~.~’ l2 Applications

EXAFS has been used to elucidate the structure of adsorbed atoms and small molecules on surfaces; electrode-dectrolyte interfaces; electrochemically produced solution species; metals, semiconductors, and insulators; high-temperature superconductors; amorphous materials and liquid systems; catalysts; and metalloenzymes. Aspects of the applications of EXAFS to these (and other) systems are neatly summarized in References 1-9, and will not be repeated here. It is important to emphasize that EXAFS experiments are indispensable for in situ studies of materials, particulary catalysts59 and electrochemical systems.l3 Other techniques that have been successfully employed for in situ electrochemical studies include ellipsometry, X-ray difhction, X-ray standing wave detection, Mossbauer-effect spectroscopy, Fourier-transform infrared spectroscopy, W-visible reflectance spectroscopy, Raman scattering, and radiotracer methods. Although the established electrochemical technique of cyclic voltammetry is a true in situ probe, it provides little direct information about atomic structure and chemical bonding. EXAFS spectroelectrochemistryis capable of providing such information.l 3 In this regard, thin oxide films produced by passivation and corrosion phenomena have been the focus of numerous EXAFS investigations. It is known that thin (420 A) passive films form on iron, nickel, chromium, and other metals. In aggressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditionsY8, l4 yet there is still some controversy about the exact nature of 224


Chapter 4

passive films on iron. The consensus is that the passive film on iron is a highly disordered form of y F e 0 0 H . Unfortunately, the majority of EXAFS studies of passive films have been on chemically passivated metals: Electrochemically passivated metals are of greater technological significance. In addition, the structures of passive films &et attack by chloride ions and the resulting corrosion formations have yet to be thoroughly investigated with EXAFS.

Conclusions Since the early 197Os, the unique properties of synchrotron radiation have been exploited for EXAFS experiments that would be impossible to perform with conventional sources of X-radiation. This is not surprising given that high-energy electron synchrotrons provide 10,000 times more intense continuum X-ray radiation than do X-ray tubes. Synchrotron radiation has other remarkable properties, including a broad spectral range, from the infrared through the visible, vacuum ultraviolet, and deep into the X-ray region; high polarization; natural collimation; pulsed time structure; and a small source size. As such, synchrotron radiation facilities provide the most useid sources of X-radiation available for FXAFS. The hture of EXAFS is closely tied with the operation of existing synchrotron radiation laboratories and with the development of new ones. Several facilities are now under construction throughout the world, including two in the USA (APS, Argonne, IL, and ALS, Berkeley, CA) and one in Europe (ESRF, Grenoble, France). These facilities are wholly optimized to provide the most brilliant X-ray beams possible-10,000 times more brilliant than those available at current facilities! The availability of such intense synchrotron radiation over a wide range of wavelengths will open new vistas in EXAFS and materials characterization. Major advances are anticipated to result from the accessibility to new frontiers in time, energy, and space. The tremendous brilliance will facilitate time-resolved EXAFS of processes and reactions in the microsecond time domain; high-energy resolution measurements throughout the electromagnetic spectrum; and microanalysis of materials in the submicron spatial domain, which is hundreds of times smaller than can be studied today. Finally, the new capabilities will provide unprecedented sensitivity for trace analysis of dopants and impurities. Related Articles in the Encydopedia

NEXAFS, EELS, LEED, Neutron Diffraction, AES, and X P S

References 'I

4.2

EUESSpectroscopy: TechniquesandApptications.(B. IC Teo and D. C. Joy, eds.) Plenum, New York, 1981. Contains historical items and treatments of EXELFS, the electron-scatteringcounterpart of EXAFS. EXAFS

225

I? A. Lee, I? H. Citrin, I? Eisenberger, and B. M. Kincaid. Extended X-ray Absorption Fine Structure-Its Strengths and Limitations as a Structural Tool. Rev. Mod. Phys. 53,769, 1981. 3 W S K Proceedings of the Fifth International Conference on X-ray Absorption Fine Structure. (J.M . de Leon, E. A. Stern, D. E. Sayers,Y. Ma, and J. J. Rehr, eds.) North-Holland, Amsterdam, 1989. Also in Pbysica B. 158, 1989. ‘‘Report of the InternationalWorkshop on Standards and Criteria in X-ray Absorption Spectroscopy” (pp. 70 1-722) is essential reading. 4 EXAFS and Near E k e structure IV Proceedings of the International Confmence. (I? Lagarde, D. Raoux, and J. Petiau, eds.) /. De Physique, 47, Colloque C8, Suppl. 12, 1986, Volumes 1 and 2. 5 EXAFS and Near U g e Structure III. Proceedings of an International Conference. (K. 0.Hodgson, B. Hedman, and J. E. Penner-Hahn, eds.) Springer, Berlin, 1984. 6 EXAFS and Near Edge Structure. Proceedings of the International Con@ence. (A. Bianconi, L. Incoccia, and S. Stipcich, eds.) Springer, Berlin, 1983. 7 X-Ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS a n d M € S . (D. C.Koningsberger and R Prins, e&.) Wiley, New York, 1988. 8 Structure of Surhces and Interfaces as Studied Using Synchrotron Radiation. Faraday Discurrions Chem. Sac. 89,1990. A lively and recent account of studies in EXAFS, NEXAFS, SEXAFS, etc. s Applications ofSynchrotronRadiation. (H. Winick, D. Xian, M. H. Ye, and T. Huang, eds.) Gordon and Breach, New York, 1988, Volume 4. F. W. Lytle provides (pp. 135-223) an excellent tutorial survey of experimental X-ray absorption spectroscopy. i o H. Winick and G. I? Williams. Overview of Synchrotron Radiation Sources World-wide. SynchrotronRadiation Nms. 4,23, 1991. 11 NationalSynchrotronLight Source User?Manual: Guide to the VUVandXRay Beam Lines. (N. E Gmur ed.) BNL informal report no. 45764, 1991. 12 B. K. Teo. EXAFS: Basic Principlesand Data Ana&sis. Springer, Berlin, 1986. 13 L. R Sharpe, W. R Heineman, and R. C. Elder. EXAFS Spectroelectrochemistry. Chem. Rev. 90,705,1990. 14 Pmsivity ofMetah and Semiconductors. ( M . Froment, ed.) Elsevier, Amsterdam, 1983. 2

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4.3

SEXAFS / NEXAFS Surface Extended X-Ray Absorption Fine Structure and Near Edge X-Ray Absorption Fine Structure DAVID NORMAN

Contents Introduction Basic Principles of X-Ray Absorption Experimental Details SEXAFS Data Analysis and Examples Complications NEXAFS Data Analysis and Examples Conclusions

Introduction

SEXAFS is a research technique providing the most precise values obtainable for adsorbate-substrate bond lengths, plus some information on the number of nearest neighbors (coordination numbers). Other methods for determining the quantitative geometric structure of atoms at surfaces, described elsewhere in this volume (e.g., LEED, RHEED, MEIS, and RBS), work only for single-crystal substrates having atoms or molecules adsorbed in a regular pattern with long-range order within the adsorbate plane. SEXAFS does not suffer from thgse limitations. It is sensitive only to local order, sampling a short range within a few A around the absorbing atom. SEXAFS can be measured from adsorbate concentrations as low as +0.05monolayers in fivorable circumstances, alrhough the detection limits for routine use are several times higher. By using appropriate standards, bond lengths can be determined as precisely as f O . O 1 A in some cases. Systematicerrors often make the accu4.3

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racy much poorer than the precision, with more realistic estimates of f0.03 A or worse. NEXAFS has become a p o w e f i technique for probing the structure of molecules on surfaces. Observation of intense resonances near the X-ray absorption edge can indicate the type of bonding, On a flat,&s the way in which the resonances vary with angle of die specimen can be analyzed simply to give the mokcdar orientation, which is precise to within a few degrees. The energies of resonances allow one to estimate the intramolecular bond length, often to within k0.05 k Usefd NFXAFS can be seen fbr concentrations as low as +0.01 monolayer in hvorable cases. The techniques can be applied to almost any adsorbate on almost any type of solid samplemetal, semiconductor or insulator. Light adsorbat-y, fiom C through A-are more difficult to study than heavier ones because their absorption edges occur at low photon energies that are technically more difficult to produce. The technique samples all absorbing atoms of the same type, and averages over them, so that good structural infbrmation is obtained only when the adsorbates uniquely occupy equivalent sites. Thus it is not easy to examine clean s k e s , where the EXAFS signal fiom surfice atoms is overwhelmed by that from the bulk. The best way to study such samples is with X rays incident on the sample at a grazing angle so that they interact only in a region dose to the surface: by varying the angle, the probing depth can be changed somewhat. The reviews of SEXAFS and NEXAFS'-5 should be consulted h r more details. Basic Principles of X-ray Absorption

The physical processes of X-ray absorption are depicted schematically in Figure 1. The energies of discrete core levels are uniquely determined by the atom type (as in XPS or AES), so tuning the photon energy to a particular core level gives an atomSpecific probe. When the photon energy equals the binding energy of the electron in a core level, a strong increase in absorption is seen, which is known as the absorption edge. The absorbed photon gives its energy to a photoelectron that propagates as a wave. In a molecule or solid, part of this photoelectronwave may be badzscattered from neighboring atoms, the backscatteredwave interfeting constructivelyor desuuctivelywith the outgoing wave.. Thus one gets a spectrum of absorption as a function of photon energy that conmins wiggles (EXAFS) superimposed on a smooth background. The amplitude of the FXWS wiggles depends on the number of neighbors, the strengh of their scatteringand the static and dynamic disorder in their position. The frequency of the EXAFS wiggles depends on the wavevector )of the photoelectrons (related to their kinetic energy) and the distance to neighboring atoms. The fiequency is inversely related to the nearest neighbor separation, with a short distance giving widely spaced wiggles and vice versa. 228


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Auger PhotoeIertmnt telectrnn kcuurn Fermi.lerel

NEXAFS

tcelectrnn

MYC

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Figure 1

"L

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Fluorescent photon

-

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I

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photon energy

Basic physical principles of X-ray absorption. As in XPSIESCA, absorption of a photon leads to emission of a photolectron.This photoelectron, propagating as a wave, may be scattered from neighboring atoms. The backscattered wave interferes constructively or destruetively with the outgoing wave, depending on its wavelength and the distance to neighboring atoms, giving wiggles in the measured absorption spectrum.

EXAFS can be used to study surfaces or bulk samples. Ways of making the technique surface-sensitive are spelled out below. EXAFS gives a spherical average of information in a shell around an absorbing atom. For an anisotropic sample with a polarized photon beam, one gets a searchlight effect, where neighbors in directions along that of the polarization vector E (perpendicular to the direction of the X rays) are selectively picked out. For studies on flat surfaces the angular variation of the EXAFS intensity is one of the best methods of identifying an adsorption site. The form of the backscattering amplitude depends on atomic number, differing between atoms in different rows of the periodic tableY5and this helps one to determine which atoms in a compound are nearest neighbors. Phase Shifts

When an electron scatters from an atom, its phase is changed so that the reflected wave is not in phase with the incoming wave. This changes the interference pattern and hence the apparent distance between the two atoms. Knowledge of this phase shift is the key to getting precise bond lengths from SEXAFS. Phase s h i h depend mainly on which atoms are involved, not on their detailed chemical environment, and should therefore be transferable from a known system to unknown systems. The phase shifts may be obtained from theoretical calculations, and there are published tabulations, but practically it is desirable to check the phase shifts using 4.3

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model compounds: the idea is to take a sample of known composition and crystallography, measure its EXAFS spectrum and analyze it to determine a phase shift $. The model compound should ideally contain the same absorber and backscatterer atoms as the unknown, and in the same chemical state. If this is not possible, the next best option is to use a model whose absorber and a backscatterer are neighboring elements in the periodic table to those in the unknown sample, although for highest precision the backscatterer should be the same as in the unknown. One must be sure of the purity of the model compound. It may have deteriorated (for example, by reaction or water absorption), its surface may not have the same composition as the bulk, or it may not be of the correct crystallographic phase. It is tempting to use single crystals to be sure of the geometric structure, but noncubic crystals give angle-dependent spectra. The crystallography of any compound should be checked with XRD. ExperimentalDetails

There are several ways to make a SEXAF/NEXAFS measurement surface sensitive. i By using dispersed samples, the surface-to-bulk ratio is increased, and standard methods of studying “bulk” samples will work (see the article on EXAFS). 2

By making the X rays incident on the sample at shallow angles (usuallya fraction of a degree), they see only the near-surface region, some 20-50 A deep. The angle of incidence can be varied, allowing crude depth profiling, but the penetration is crucially dependent on the flatness of the reflecting surface, and large homogeneous samples are needed. This is potential1y.a useful technique for studying buried interfaces, where the signal will come predominantly from the interfice if the substrate is more dense than the overlayer.This method has been little tried in the soft X-ray region but should work well, since the critical angle is larger than for hard X rays.

3

Since X-ray absorption is an atom-specific process, any atoms known to be, or deliberately placed, on a solid consisting of different atoms can be studied with high sensitivity. The absorption may be monitored via a secondary decay process that is surficesensitive, such as the emission of Auger electrons, which have a well-defined energy and a short mean free path.

4

X-Ray Sources

The only X-ray source with sufficient intensity for surface measurements is synchrotron radiation. Synchrotron radiation is white light, including all wavelengths from the infrared to X rays. A spectroscopy experiment needs a particular wavelength (photon energy) to be selected with a monochromator and scanned through 230


Chapter 4

the spectrum. For EXAFS, a range is needed of at least 300 eV above the absorption edge that does not contain any other edges, such as those from coadsorbates, the substrate, or from higher order light (unwanted X rays from the monochromator with two, three, or more times the desired energy). NEXAFS needs a clear range of perhaps 25-30 eV above the edge. Perusal of a table of energy levels is essential.6 Photon energies from about 4 keV to 15 keV are easiest to use, where X-ray windows allow sample chambers to be separated from the monochromator. Energies below about 1800 eV are technically the most difficult, requiring ultrahigh vacuum monochromators directly connected to the sample chamber. K edges are easiest to interpret, but L2,3 edges can be used: line widths are much broader at L, edges, and states such as M, may have an absorption edge too wide to be usable for EXAFS.

Detection Methods

The experiment consists of measuring the intensity of photons incident on the sample, and the proportion of them that is absorbed. Most SEXAFS experiments detect the X-ray absorption coefficient indirectly by measuring the fluorescence or Auger emission that follows photon absorption. (See the articles on AES and XRF.) The various electron or photon detection schemes should be tested to see which one gives the best data in each case. Measuring all electrons, the total electron yield (TEY), or those in a selected bandpass, the partial electron yield (PEY), will give Fluores. higher signals but poorer sensitivity than the Auger electron yield (MY) cence yields (FYs) are low for light elements, so their measurement usually gives weak signals, but the background signal is usually low, in which case FY will give high sensitivity. FY is the technique of choice for insulating samples that may charge up and confuse electron detection. FY also allows for experiments in which the sample is in an environment other than the high vacuum needed for electrons. With suitable windows, surface reactions may be followed in situ, for instance in a high-pressure chamber or an electrochemical cell, although this type of work is yet in its i n h c y .

Electron Excitation

The advantages of SEXAFVNEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-like phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence7that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more taxing than SEXAFS. 4.3

SEXAFS/NEXAFS

231

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Figure 2

Surface EXAFS spectra above the Pd b-edge for a 1.5 monolayer evaporated film of Pd on Sill 111 and for bulk palladium silicide, P&Si and metallic Pd.

SEXAFS Data Analysis and Examples

Often a comparison of raw data directly yields usell structural information. An example is given in Figure 2,which shows SEXAFS spectra*above the palladium L2 edge for 1.5 monolayers of Pd evaporated onto a Si (1 11) surface, along with pure Pd and the bulk compound Pd2Si. It is clear just from looking at the spectra and without detailed analysis that the thin layer of Pd reacts to give a surface compound similar to the palladium silicide and completely different from the metallic Pd. By contrast, a thin layer of silver, studied in the same experiment, remains as a metallic Ag overlayer, as judged from its SEXAFS wiggles. Fourier Transformation

One of the major advantages of SEXAFS over other surface structural techniques is that, provided that single scattering applies (see below), one can go directly from the experimental spectrum, via Fourier transformation, to a value for bond length. The Fourier transform gives a red space distribution with peaks in IF(R)I at distances R- 9. Addition of the phase shifi, 9, then gives the true interatomic distance. Figure 3 shows how this methodg is applied to obtain the 0-Ni distance in the half-monolayer structure of oxygen absorbed on Ni (100). The data, after back232


Chapter 4

20

,

15

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n

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Y 103 4 5 6 7 8 9 1 0

Figure 3

The modulus of the Fourier transform of the SEXAFS spectrum for the halfmonolayer coverage on Ni(100)The SEXAFS spectrum itself is shown in the inset with the backgroundremoved.

ground subtraction, yield a Fourier transform dominated by a single peak at R= 1.73 A. Correcting for the phase shift derived from bulk NiO, a nearest neighbor distance of R a ~ =i 1.98 A is obtained. Fourier transforms cannot be used if shells are too dose together, the minimum separation AR being set by the energy range above the absorption edge over which A fix SEXAFS. A us& application of the Fourier data are taken, typically ~0.2 technique is to filter high-frequencynoise from a spectrum. This is done by putting a window around a peak in R-space and aansforming back into &space: each shell may be filtered and analyzed separately, although answers should always be checked against the original unfiltered spectrum. Curve fitting

The beauty of using photons is that their absorption is easily understood and exactly calculable, so that structural analysis can be based on comparisonsof experimental data and calculated spectra. Statistical confidence limitscan easily be computed, although the systematic errors will often be much greater than the random errors. An example of data analysis by curve fitting is depicted in Figure 4 fix the system of 44 monolayer of C1on Ag (111).l0 The nearest neighbor Cl-Ag (2.70 A) and Cl-Cl(2.89 A) shells are so dose in distance that they cannot be separated in a Fourier transform approach, but they are easily detected here by the fact that their atomic backscattering factors vary differently with energy, thus influencing the overall shape of the spectrum. 4.3

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233

7.0

5.0

9.0

k

Figure 4

The EXAFS function X(k),weighted by 3,experimentaldata for % monolayer of CI on Ag(l11) (solidline), with the best theoretical fit (dashedline)fr?m the 1east;squares cuve fitting metho! with neighborsas distances of 2.70 A (Ag), 2.89 A (CL),3.95 A (Ag)and 5.00 A (CI).

Complications

The simple theory assumes single scattering only, in which electrons go out only from the absorber atom to a backscatterer and back, rather than undertaking a journey involving two or more scattering atoms. Such multiple scattering may sometimes be important in EXAFS, especially when atoms are close to collinear, giving wrong distances and coordination numbers. With modern, exact theories of EXAFS one can deal with multiple scattering, but it is complicated and time-consuming, and a unique analysis may be impossible. However, nearest neighbor information can never be affected by multiple scattering, since there is no possible electron path shorter than the direct single scattering route. EXAFS analysis usually assumes a shell of neighbors at a certain distance, with a Gaussian (normal) distribution around that distance to cope with the effects of disorder, both static (positional) and dynamic (vibrational). Static disorder arises where, even at zero temperature, a range of sites is occupied, as found particularly with amorphous or glassy samples. EXAFS samples directly the distance between nearby atoms and thus measures correlated motion, giving a disorder (DebyeWaller) factor smaller than that derived from long-range diffraction techniques like XRD or LEED. Vibrational amplitudes at a surfice usually differ from those in the bulk, and SEXAFS spectra measured at different angles have been used to reveal surface dynamics, resolving vibrations parallel and perpendicular to a single-crystal surface. 234


Chapter 4

The assumption of harmonic vibrations and a Gaussian distribution of neighbors is not always valid. Anharmonicvibrations can lead to an incorrect determination of distance, with an apparent mean distance that is shorter than the real value. Measurements should preferably be carried out at low temperatures, and ideally at a range of temperatures, to check for anharmonicity. Model compounds should be measured at the same temperature as the unknown system. It is possible to obtain the real, non-Gaussian, distribution of neighbors from EXAFS, but a model for the distribution is needed" and inevitably more parameters are introduced. Some of these complications can lead to an incorrect structural analysis. For instance, it can be difficult to tell whether one's sample has many nearest neighbors with large disorder or fewer neighbors more tightly ddined. Analysis routines are available at almost all synchrotron radiation centers: curve fitting may be the best method because most of the k t o r s affectingthe spectrum vary with energy in a different way and this kdependence allows them to be separated out. A curved-wave computational scheme can be especially useful for analyzing data closer to the absorption edge.

NEXAFS Data Analysis and Examples Chemical Shifts and PmEdge Features

The absorption edge occurs when the photon energy is equal to the binding energy of an electron core level. Shifts in the position of the edge are caused by small differences in the chemical environment, as in ESCA (XPS). If one needs to know the exact energy of the edge,perhaps for comparison with other published data, then a model compound with a calibrated energy should be measured under the same conditions as the unknown. Features may be seen before the absorption edge, most obviously in transition metals and their compounds. These small peaks are characteristic of local coordination (octahedral, tetrahedral or whatever): their intensity increases with oxidation state. Atomic Adsorbates

The NEXAFS region near an absorption edge is usually discarded in an EXAFS analysis because the strong scattering and longer mean free path of the excited photoelectron give rise to sizable multiple-scattering corrections. For several atomic adsorbates NEXAFS has been modeled by complicated calculations, which show that scattering involvingaround 30 atoms, to a distance >5.0 A horn the absorbing atom, contributes to the spectrum. This makes interpretation difficult and not usefd for practical purposes, except possibly for fingerprinting different adsorption states. 4.3

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235

200

Figure 5

290 300 Photon energy lev)

310

NEXAFS spectra above the C K-edge for a saturation coverage of pyridine &H,N on Pt(l111, measured at two different polarisation angles with the Xray beam at normal incidence and at 20" to the sample surface.

Molecular Adsorbates-orientation

For molecules, NEXAFS often contains intense resonances that dwarf the effects of atomic scattering in the spectrum. These resonances arise from states that are localized in space within the molecule, rather than being spread out and shared between various atoms: they are thus mainly characteristic of the molecule itself and only weakly affected by differences in the way the molecule may be bonded to the surhce. Despite the technical difficulties, most NEXAFS work has been done at the carbon K-edge. An example is depicted in Figure 5, which shows NEXAFS for a saturation coverage of pyridine C,H,N on Pt (11l), measured at different angles to the photon beam.12 Peak A is identified as a IC resonance, arising from transitions from the C 1s state to the unfilled d molecular orbital. Peak B comes from CO impurity. Peaks C and D are transitions to Q shape resonances that lie in the plane of the molecule. The variation of intensity of the II and CJ resonances with polarization angle gives the molecular orientation, each peak being maximized when the polarization vector E lies along the direction of the orbital. The R intensity is great236


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Chapter 4

est when E is parallel to the surfice (e = 90°), so the K orbitals must lie pardel to the surface. Therefore the pyridine molecule must stand upright on the Pt (1 11) surface. NEXAFS alone tells us only the orientation with respect to the top plane of the substrate, not the detailed bonding to the individual atoms, nor which end of the molecule is next to the surface: this detailed geometry must be determined from other techniques. There may be deviations from the perfect angular dependence due to partial polarization of the X rays or to a tilted molecule. This can be investigated by analysis of the intensities of the resonances as a function of angle. Measuring the intensity of NEXAFS peaks is not always straightforward, and one has to be careful in removing experimental artifacts from the spectrum and in subtracting the atomic absorption background, for which various models now exist. Detailed analysis is not always needed, for instance the mere observation of an: resonance can be chemically useful in distinguishing between 'IC and di-obonding of ethylene on surfaces. NEXAFS can be applied to large molecules, such as polymers and LangmuirBlodgett films. The spectra of polymers, such as those13 depicted in Figure 6, contain a wealth of detail and it is beyond the current state of knowledge to assign all the peaks. However, the sharper, lower lying ones are attributed to TC*molecular orbital states. Changes in these features were observed after deposition of submonolayer amounts of chromium, from which it is deduced that the carbonyl groups on the polymers are the sites for initial interaction with the metal overlayer. It has been suggested4 that most examples of molecular adsorbate NEXAFS may be analyzed with quite simple models that decompose complex molecules into building blocks of diatoms or rings. Intramolecular Bond Length

The energies of shape resonances often seem inversely related to the intramolecular bond length, with a long bond giving a o resonance dose to threshold and a shorter bond showing a peak at higher energy.4 This effect has been demonstrated for many small molecules, although some do not fit the general trend. A mathematical relationship has been derived to allow estimates of bond length, but with the current state of knowledge it seems safest to restrict its use to diatomic molecules or ligands. With this procedure, intramolecular chemisorption bond lengths can be determined to an accuracy of f0.05 k Conclusions

X-ray absorption spectroscopy is an important part of the armory of techniques for examining pure and applied problems in surface physics and chemistry. The basic physical principles are well understood, and the experimental methods and data analysis have advanced to sophisticated levels, allowing difficult problems to be solved. For some scientists the inconvenience of having to visit synchrotron radia4.3

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J

205

290

245

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305

Photon energy ( e V

Figure 6

NEXAFS spectra above the C K-edge for the polymers PMPO poly (dimethyl phenylene oxide], PVMK poly (vinyl methyl ketone). PMDA-MBCA PI poly (pyromelliiimido 4, 4-methylene biGcyclohexyl amine) and PMDA-ODA PI poly (pyromelliiimido oxydianiline).

tion centers is outweighed by the unique surface structural information obtainable fiom SEXAFS/NEXAFS. Nevertheless, although they are powerful techniques in the hands of specialists, it is difficult to foresee their routine use as analytical tools. A database of model compound spectra and a better understanding of complex molecules would help the inexperienced practitioner. More dilute species could be studied by brighter synchrotron radiation sources. An obvious experimental improvement would be to use a polychromatic energy-dispersive arrangement for speedier data collection. In such a scheme the X rays are dispersed across a sample so that photons having a range of energies strike the specimen, and a detection 238


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Chapter 4

method has to be used that preserves the spatial distribution of the emitted electrons. Currently available photon fluxes are such that collection times less than or about one second should then be obtainable for a NEXAFS spectrum. Related Articles in the Encydopedia

AES, EXAFS, LEED/RHEED, X P S , and XRF References 1

D. Norman. J. Pbys. C: Solidstate Pbys.19,3273, 1986. Reprinted with an appendix bringing it up to date in 1990 as pp. 197-242 in Current Topics in Condenred Matter Specmscopp Adam Hilger, 1990. An extensive review of SEXAFS and NEXAFS, concentrating on physical principles.

2

I? H. Citrin.J. Pbys.Coll.C8,437,1986. Reviews all SEXAFS work up to

1986, with personal comments by the author. 3 X-RayAbsorption: PrincipLes, Applications, Techniques of EAXES, SExAE;S a n d M E S . (D.C. Koningsberger and R Prins, Eds.) Wiley, New York, 1988.The best book on the subject. Especially relevant is the chapter by J. Stohr, which is a comprehensiveand readable review of SEXAFS. 4 J. Stohr. NEXAFS Spectroscopy Springer-Verlag,New York, 1992. A book reviewing everything about NEXAFS. 5 D. Norman. In Physical Metbod of Chemistry Wiley-Interscience, New York, in press. Practical guide with emphasis on chemical applications. 6 The X-RayData Booklet. Lawrence Berkeley Laboratory, Berkeley, is an excellent source of information. 7 M. de Crescenzi. Su$ Sei. 162,838, 1985. 8 J. Stohr and R Jaeger. J. f i e . Sei. Tecbnol. 21,619,1982. 9 L. Wenzel, D. h a n i t i s , W. D a m , H. H. Rotermund, J. Stohr, K. Baberschke, and H. Ibach. P l y . Rev. B. 36,7689, 1987. i o G. M. Lamble, R S. Brooks, S. Ferrer, D. A. King, and D. Norman. Pbys. Rev. B. 34,2975,1986 11 T. M.Hayes and J. B. Boyce. Solidstate Pbys.37,.173, 1982. Good background reading on EXAFS. 12 A. L. Johnson, E. L. Muetterties, and J. Stohr. ]. Cbm. Soc. 105,7183, 1983. 13 J. L. Jordan-Sweet, C. A. Kovac, M. J. Goldberg, and J. F. Mom.]. Cbm. Pbys.89,2482, 1988.

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4.4

XPD and AED X-Ray Photoelectronand Auger Electron Diffraction BRENT D. HERMSMEIER

Contents

Introduction Basic Principles Experimental Details IllustrativeExamples Special Topics Conclusions

Introduction

X-ray Photoelectron DifK-aaion (XPD) and Auger Electron Difiaaion (AED) are well-established techniques for obtaining structural information on chemically specific species in the surface regions of solids. Historically, the first XPD effects were observed in single crystals some 20 years ago by Siegbahn et al.' and Fadley and Bergstrom2A short time later, others3began to quantify the techni ue as a surface and SUrfac-Radsorbatestructural tool and, more recently, Egelhoff applied AED to metal-on-metal systems to determine growth modes and shallow interface structures, which has strongly influenced the current expansion to materials research. In general, these pioneering studies have introduced XPD and AED to areas like adsorbate site symmetry, overlayer growth modes, s d c e structural quality, and element depth distribution^,^^ any one of which may be a key to understanding the chemistry or physics behind a measured response. Studiesas widely separated as the initial stages ofmetallic corrosion, intehce behavior in epitaxial thin fdms, and semiconductor surface segregation have already profited from XPD and AED experiments. A broad range of research communitie-talysis, semiconductor,

1

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corrosion, material science, magnetic thin film, and packaging, stand to benefit from these diffraction studies, since surfaces and interfaces govern many important interactions of interest. XPD and AED are used primarily as research tools, but, given the hundreds of X P S and AES systems already in use by the aforementioned research communities, their move to more applied areas is certain. Adaptation of existing XPS and AES instruments into XPD and AED instruments is straightforward for spectrometersthat are equipped with an angle resolved analyzer. Traditionally,XPD and AED instruments were developed by individuals to address their specific questions and to test the limits of the technique itself. Today, surface science instrument companies are beginning to market XPD and AED capabilities as part of their multi-technique spectroscopic systems. This approach has great potential for solving both a broad range of problems as is typically found in industrial laboratories and in studies that intensely focus on atomic demils, as ins often found in university laboratories. Key to obtaining quality results, whichever the mode of operation, are in the speed of data acquisition, the angular and energy resolution, the accessible angular range, and the capability to meaningfully manipulate the data. The reader is urged to review the XPS and AES articles in this Encyclopedia to obtain an adequate introduction to these techniques, since XPD and AED are actually their by-products. In principle two additions to X P S and AES are needed to perform diffraction studies, an automated two-axis sample goniometer and an angle-resolved analyzer. Ultrahigh-vacuum conditions are necessary to maintain surface cleanliness. Standard surface cleaning capabilitiessuch as specimen heating and Ar+ sputtering, usually followed by sample annealing, are often needed. Sample size is rarely an issue, especially in AED where the analysis area may be as small as 300 A using electron field emitter sources. Excellent reviews of XPD and AED have been published by F a d l 4 1 and are strongly recommended for readers needing information beyond that delineated here.

Basic Principles The diffraction mechanisms in XPD and AED are virtually identical; this section will focus on only one of these techniques, with the understanding that any conclusions drawn apply equally to both methods, except where stated otherwise. XPD will be the technique discussed, given some of the advantages it has over AED, such as reduced sample degradation for ionic and organic materials, quantification of chemical states and, for conditions usually encountered at synchrotron radiation facilities, its dependence on the polarization of the X rays. For more details on the excitation process the reader is urged to review the relevant articles in the Encyclopedia and appropriate references in Fadle~.~. 4.4

XPD and AED

241

Scatterlng Concept Analyzer Solld Angle k 3 O )

hv Conrtructlva

Intenelty

urface Scattered Wave

Photoelactron Emmlrlon Asymmetry

Figure 1

Simplistic schematic illustration of the scattering mechanism upon which Xray photoelectrondflraction (XPDI is based. An intensity increase is expected in the forward scattering direction, where the scattered and primary waves constructively interfere.

XPD is a photoelectron scattering process that begins with the emission of a spherical electron wave created by the absorption of a photon at a given site. This site selectivity allows XPD to focus on specific elements or even on different chemical states of the same element when acquiring diffraction data. The excitation process obeys dipole selection rules, which under special conditions may be used to enhance regions or directions of interest by taking advantage of photoelectron emission asymmetries in the emission process; for example, enhanced surface or bulk sensitivitycan be obtained by aligning the light’s electricfield vector to be parallel or perpendicular to the sample’s surfice plane, respectively. This flexibility, unfortunately, is not availablein most spectrometers because the angle between the excitation source and the analyzer is fmed. The spherical photoelectron wave propagates from the emitter, scatters off neighboring atoms, and decays in amplitude as Vr. The scattering events modifjr the photoelectron intensity reaching the detector relative to that of the unscattered portion, or primary wave. A physical picture of this is given in Figure 1, where a spherical wave propagating outward from the emitter passes through a scattering potential to produce a spherically scattered wave that is nearly in phase with the primary wave in the forward direction. Since the primary and scattered waves have only a slight phase shift in this direction the two waves can be thought of as constructively interfering. However, since both waves 242


...

Chapter 4

are spherical and have different origins, they will tend to interfere destructively in off-forward directions. This interference process results in increased intensity for emission geometries in which rows of closely spaced atoms become aligned with the entrance axis to the analyzer. The diffraction pattern itself is simply the mapping of the intensity variations as a function of polar or azimuthal angles. Angular scans can be obtained by rotating the sample while leaving the analyzer fmed, or by rotating the analyzer and leaving the sample fmed. In either case, an intensity profile is obtained for a given core-level transition. The recipe is therefore quite simple. First, to determine whether the sample is structurally ordered, one simply looks for any intensity variation as a function of angle; if found it can be concluded that there exists some kind of order over the probing length of the photoelectron. Second, to identify a low-index crystallographic direction, one looks for maxima along various logical or predetermined directions, such as those associated with cleavage planes or previously identified by X-ray diffraction. Usually this confirmation can be done by directly monitoring the intensity with a ratemeter if the specimen is reasonably well ordered. Third, to determine the symmetry of the structure, often the main goal in XPD, one collects several polar and azimuthal scans to correlate the appearance of diffraction peaks at measured angles to suspected Bravais lattice structures having near-neighbor atoms at similar angles. Fourth, one monitors changes in the diffraction features as a function, for example, of sample temperature or of overlayer thickness. This is particularly informative when comparing absorbate or overlayer symmetries with that of the substrate in a fingerprint analysis mode. Although the intensities are dominated by forward scattering processes, a detailed understanding must consider contributions to the detected intensity by all of the scattering atoms within several lattice constants of the emitter. To simulate such a scatteringprocess, a kinematical or single-scattering approach is sufficient if the electron’s kinetic energy is greater than 150 eV, and if it is not necessary to fully understand the fine structure in the diffiaction pattern5’ Complicated multiple-scattering calculations can also aid in the quantification of XPD data by more accurately addressing intensity anisotropies and improving identification of the fine structure. But more often than not, rnultiple-scattering effects contribute little to the basic understanding of the structure, and thus will not be discussed further. Single-scattering results wii be displayed along with experimental data, and compared to geometric arguments for resemblance. Unlike more common elecrron diffraction methods, such as LEED and M E E D , XPD is dominated by near-neighbor interactions averaged over a very short time scale. The l/r decay of a spherical wave, coupled with a short mean free path for electrons in solids (due to inelastic scattering energy losses), uniquely allows XPD to probe the local, or short-range, order about an emitter. This is simplified hrther by the incredibly short times involved in the scatteringprocess. Only IO-’’ seconds are required for a photoelectron to experience a scattering event

‘

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XPD and AED

243

while typical crystal fluctuations, which can average out short-range order effects, occur on a much longer time scale. In essence, the scattering events can be thought of as a snapshot of the local crystal order. A brief comment needs to made to clear up some misleading information that has entered the literature concerning the scattering mechanism upon which XPD and AED are based. Frank et al.' have proposed what they believe to be a new technique capable of solving surfice structure problems. The technique, which they is based on an arguhave termed angular distribution Auger microscopy (ADAM), ment claiming that emitted Auger electrons are solid particles that are blocked by neighboring atoms in the solid. This interpretation is in direct contradiction with the scatteringpicture presented in the basic principles section which is based on the quantum mechanical wave nature of electrons, a picture for which there is overwhelming evidence from theoretical and experimental comparisons. In light of this, no further consideration will be given to ADAM in this article. Experimental Details

The specimen to be studied usually will be an ardered solid, such as a single crystal or a textured sample, that is rigidly mounted face-up on the goniometer. Prior knowledge of the crystal orientation is greatly beneficial. In most instances the samples or substrates are aligned with a low-index direction normal to the surface by means of standard X-ray diffraction methods, e.g., by back-reflection h u e or 8-28 scans. The surfice plane of the sample should lie in, or as dose as possible to, the polar rotation axis; the maximum o s e t should be 2.5 mm. An often more critical alignment is that needed to get the su&ce normal and the azimuthal rotation axis to coincide. This will minimize crystal "wobble," thus minimizing ambiguous diffraction effects (usually apparent as a sloping background) that are accentuated at grazing emission angles, where signal intensities are dominated by an instrumental function highly responsive to slight changes in polar angle. Proper azimuthal alignment is obtained by centering a surfice-reflected laser beam to within f 0.25" during azimuthal rotation. Polar rotation should have a range of at least 120" to include both grazing and surfice-normal emissions directions and the azimuthal range should have a minimum of 200°, preferably 380°,to allow for the full rotational symmetry. Since diffraction data is angle dependent, an angle-resolved analyzer is necessary to discriminate electron trajectories, allowing only those electrons with similar emission directions to reach the detector. The practical upper limit for the acceptance solid-angle is approximately f 12", with f 3" being a more common value. (There have been some high angular-resolution studies done at f 1Soshuwing diffraction fine structure that led to a more quantitative description of the observed structure, however t h i s will not be necessary for routine structure determinations.) Increasing the angular resolution is usually a straightforward task that involves the 244


Chapter 4

physical placement of an aperture or an array of cylinders in front of the electronanalyzing optics. Or, if an electron lens is included, voltage adjustments to the lens elements may act to effectively reduce the acceptance solid angle. This, as an example, has been done by the author on a VG MICROLAB I1 spectrometer, which has a two-element lens. Here, when the front elements are powered and the back elements are grounded, a f 12" solid angle results, while a k 4" solid angle may be achieved by powering the back elements and grounding the front. In either mode, the implementation of a reduced solid angle and sample rotation is a relatively easy and inexpensive procedure, and may be accomplished with many existing electron spectrometers. Illustrative Examples Surface adsorbates

From environmental to packaging to catalysis issues, the need to understand how molecules interact chemically and bond to a surface is paramount. XPD is an extremely good candidate for investigating adsorbatesubstrate interactions because chemical shifts in the core-level transitions can lead to the identification of a specific species, and the scattering of core-level photoelectrons can lead to the determination of the structure in which they exist. Consider the initial interaction of gaseous CO on room temperature Fe (O01).8At this temperature Fe (001) has a bcc lattice structure with a fourfold symmetric surface. At a coverage of less than a monolayer, it was known that the CO adsorbs to the Feyresiding in fourfold hollow sites with only the C making direct contact with the Fe. The orientation of the C-0 bond remained a question. It was proposed that this early stage of CO coverage on Fe (001) produced an intermediary state for the dissociation of the C and 0,since that the CO bond was tilted with respect to the surface normal, unlike the upright orientation that CO was found to possess on Ni.5 Although near-edge X-ray adsorption fine structure (NEXAFS) results measured a tilt in the CO bond, the results were not very quantitative regarding the exact angle of the tilt. XPD, on the other hand, gave the CO bond angle as 35 +2" relative to the surfice, as determined from the large forward scattering peak depicted in by the solid line Figure 2a along the (100) azimuth. Here the ordinate is plotted as the C 1s intensity divided by the 0 1s intensity. Plotting this ratio effectively removes instrumental contributions to the diffraction pattern, the oxygen atoms have no atoms above them from which their photoelectrons can scatter and thus should be featureless. The azimuthal scan shown in Figure 2b was taken at a polar angle of 35" to enhance the C 1s diffraction signal. From the fourfold symmetry and knowledge of the crystallographicorientation of the Feyit is clear that the tilt direction lies in the planes, as depicted in Figure 2c; the absence of a diffraction peak in the [lTO] polar scan shown by the dashed line in Figure 2a helps to confirm this. 4.4

XPD and AED

245

a

CO(a3)/Fe(O01) Surface

C

I

Azimuthal Angle @

=o =c

o= Fe

2.878,

1.8

Figure 2

’ ’

I 0’

I

’

’

,Im

450

I

’ I

’

I

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900

Experimental data from an early stage of CO adsorbed on Fe (001) known as the or, state: polar scans (a) of the C 1s-O 1s intensity ratio taken in two Fe (001) azimuthal planes, the (100) and the (170) (the C 1s and 0 1s electron kinetic energies are 1202 eV and 955 eV, respectively);C 1s azimuthal scan (b) taken at the polar angle of maximum intensity in (a); and geometry IC) deduced from the data.

The intensity anisotropies in Figure 2b associated with the difiaction peaks are similar in magnitude. This was found to be true also for the other two quadrants not shown here and suggests that there is no preferred tilt of the CO molecule into any one of the four quadrants. This also demonstrates the high level of sensitivity one can expect for a n XPD pattern, considering that less than a quarter monolayer of (low-2) CO molecules, i.e., only those with G O bonds pointing in the direction presented in Figure 2b, contribute to a 16% anisotropy. T o put this into perspective, a “perfect” single crystal will yield an anisotropy of about 50%. Since the diffraction features are dominated by near-neighbor scattering events, a 16% intensity change is not too surprising and M e r suggests that even though the CO molecules are tilted along four different directions they are highly ordered along each of them. This presents some practical experimental assurance of the sensitivity that can be expected from XPD or AED. Overlayem

Perhaps the best examples to illustrate the analysis strength of XPD and AED are the epitaxial growth modes of deposited overlayers. Here, the structure and chemistry of an overlayer, or the new interface, will influence the properties of the film. To control such effects, an understanding of the basic structure and chemistry is essential. Epitaxial Cu on Ni (001) is a n excellent example for demonstrating the 246


Chapter 4

Epitaxial

a

Cu on Ni[Wl]

Ekln=917eV

n

I

, \

- Expt. ---

(Egelhoff) Theory: s s c - P w cu: -

b

bulk 50 ml

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+

Nil0011 sudstraie

Figure 3

2

3 A

5

[loo]-

Experimental and calculated results (a) for epitaxial Cu on Ni (001). The solid lines represent experimental data at the Cu coverage indicated and the dashed lines represent single-scatteringcluster calculationsassuming a plane wave final state for the Cu LMM Auger electron; A schematic representation (b)of the Ni (010) plane with 1-5 monolayersof Cu on top. The arrows indicate directions in which forward scattering events should produce diffraction peaks in (a).

types of structural information that can be obtained for a metal-on-metal system. Figure 3a shows several experimental polar diffraction patterns obtaiued using the Cu LMMAuger transition, which are compared to single-scatteringcluster calculations that use plane waves to represent the emitted electrons.' XPD patterns using the Cu 2P3/2 core level would look virtually identical. Figure 3b gives a diagrammatic sketch to help interpret the origins of the diffraction features. The diagram should be viewed, layer by layer, from the top down; the angles associated with each arrow, when in full view, indicate the directions in which a diffraction peak should appear. For example, &er the second monolayer is formed three arrows are in full view, at angles of 0", 1S0, and 45". The first arrow represents an emission direction in which it is physically impossible to collect data. The other two arrows appear in the experimental data of Figure 3a only h e r a second monolayer is deposited. The appearance of these diffraction peaks with the deposition of the second monolayer is consistent with intensity maxima occurring along directions having neighboring atoms. This is then confirmed by the sudden appearance of the diffraction peak at 90" after the deposition of the third layer. 4.4

XPD and AED

247

Although the interpretation presented is conceptually inviting, caution must be taken not to extend it beyond what is reliable. For this reason, results from singlescattering duster calculations are included in Figure 3. By calculating the intensity from a hbricated atomic duster, in which atoms can be removed from the duster at will, a very accurate understanding of the relative contributions for various atoms can be made. Indeed, the peaks at 45" and 90" result almost entirely from scattering events with the near-neighbor atoms along these directions,whereas the peak at 70" gets its intensity not only from the atoms along this direction but, to some degree, also from the near-neighbor atoms at 90" and 45". One should be carell when attributing diffraction intensities to atoms in the solid that are more than a few neighbors away. Polar XPD data taken in a low-index azimuthal plane creates the opportunity to directly obtain the ca axis ratio for measuring induced lattice strain. This has been done by Chambers et al." for the Cu-Ni system mentioned above and, more recently, by the author et al." for thin Co layers in a Co-Pt superlattice. The induced strain in the overlayer alters the angles, excluding the 90" peak, at which the diffraction signals appear. The change in angle leads to a direct determination of the type and degree of strain present in the sample.

Special Topics

As is common to most techniques, XPD and AED have spawned related techniques that are too specialized to present in detail here, but the reader should be at least aware of some of the more interesting ones, which show hture possibilities. Holography

It is the goal of XPD and AED to give chemically specific structural information in ordered samples. The presentation of such data has recently been simplified from angular plots to real space images. The mechanics of the data collection procedure remains the same as in normal XPD or AED, with the additional need to acquire data over the full hemisphere of angular space above the surface, or a significant fraction thereof. The difference emerges in the final treatment, which involves a Helmholtz-Kirchoff Fourier transformation. The result is an image that apparently allows one to display the surhce structure from numerous perspectives and to directly measure the bond distances in the solid." It is important to point out, however, that using the preceding scattering theories one can get exactly the same information, minus the friendly display. Spin-Polarized Photoelectron Dii%ction

This article has been devoted to understanding the structural order at surfices and interhces through XPD and AED. However, these techniques are not limited to 248


Chapter 4

structural studies. An example can be found in studies of surface magnetic ordering. Here XPD is thought to offer a unique approach by taking advantage of the physics involved in the emission of outer core-level photoelectrons from magnetic atoms and in the electron-atom scattering process. Simply put, such photoelectrons will have multiple final states separated in energy (several eV) and spin. An energy-dispersive analyzer can thus be used to monitor spin-polarized signals. The scattering process consisting of two contributions, nonmagnetic (Coulombic) and magnetic (exchange),will affect a spin-parallel photoelectron differently from an antiparallel one. It will thus give rise to spin-polarized XPD or spin-polarized photoelectron diffraction (SPPD).12 The simplest approach for understanding SPPD is to monitor spin-split photoelectron peaks as a function of crystal temperature, as opposed to angle (as done in XPD). A relative change in intensity, beyond that expected by lattice responses to the temperature (such as Debye-Waller effects) can be attributed to a change in the magnetic order of the solid. As has been observed, a sudden change occurs in the relative photoelectron intensities at temperatures well above the bulk ordering temperature that is speculated to be a surface short-range ordering temperature. As a final point, since the spin-polarized beam of photoelectrons is referenced internally to the sample and not the spectrometer, as in numerous other spin-polarized electron experiments, it appears to be able to measure antiferromagnetic as well as ferromagnetic materials. Surface Metting

The ability of XPD and AED to measure the short-range order of materials on a very short time scale opens the door for surface order-disorder transition studies, such as the surface solid-to-"liquid" transition temperature, as has already been done for Pb and Ge. In the case of bulk Ge,l3 a melting temperature of 1210 K was found. While monitoring core-level XPD photoelectron azimuthal scans as a hnction of increasing temperature, the surface was found to show an order-disorder temperature 160' below that of the bulk. Valence Bands

The ambiguous nature of valence band peaks are well known, especially when h r ther complicated by having a multicomponent system, such as compounds and alloys. XPD has been used to offer unique insight in addressing this problem. The process is straightforward. By directly comparing the degree of similarity between core-level diffraction patterns and diffraction patterns of deconvolved valence band peaks, one can immediately determine which peaks are due to which elements.'* From here, the total density of states can be separated then into its partial densities of states. This has important implications, as the use of valence-band regions for identifjring complicated compounds increases. 4.4

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Conclusions

There is a clear need in materials science to achieve a more quantitative understanding of chemical and structural properties of surfices and interfaces. XPD and AED offer both with a relatively small investment in time and equipment, if one is using XPS or AES already. With a foundation in spectroscopy, XPD and AED are well suited for elemental analysis; XPD is capable even of giving quantitative chemical state information in many cases. Once the element or elements are identified, information on structure can be gathered. The approach emphasized in this article is the simplest and most straightforward, and consists of correlating the main experimental diffraction peaks obtained from several diffraction scans with near-neighbor atomic positions, leading to a determination of the lattice type. When an overlayer is deposited onto a substrate, there is the unique ability to obtain diffraction patterns fiom both simultaneously-measuring the quality of the lattice matching as well as the structure. As a further step, exact atomic positions can be obtained by comparing the experimental diffmction data with scattering theory. The information gained for a given amount of effort is substantial. Hundreds of electron spectrometers are already being used to pursue surface chemical information, many on crystdine and textured materials, and it seems only reasonable that a number of these could benefit further from the addition of structural analysis capabilities. Should t h i s addition be realized, one would expect that XPD and AED will move more from the basic research environments into applied areas. An example of this can be seen with the future growth of thin-film technology where XPD and AED may serve a valuable function in quality control issues by monitoring the degree of surface texturing or interface diffusion. Whatever the future may be, surfice structure and chemistry will inevitably play a key role in materials research. XPD and AED are good candidates for doing both. Related Articles in the Encyclopedia

X P S , AES, EXAFS, SEXAFS, LEED, M E E D . References

K. Siegbahn, U. Gelius, H. Siegbahn, and E. Olsen. Phys. Lett. 32A, 221, 1970. z C. S. Fadley and S. A. L. Bergstrom. Phys. Lett. 35A, 375, 1971. 3 S. &no, C. S. Fadley, N. F. T. Hall and Z. Hussein. Pbys. Rev. Lett. 41, 117,1978; S. D. Kevan, D. H. Rosenblatt, D. Denley, B.-C. Lu, and D. k Shirley. Pbys. Rev. Lett. 41,1565 (1978);D. l? Woodruff, D. Norman, B. W. Holland, N. V Smith, H. H. Farrell and M. M. Traum. Pbys. Rev. Let. 41,1130,1978. 1

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W. F. EgelhofX Pbys.Rev.B30,1052,1984; R A. Armstrong and W. E. EgelhoE Sa$ Sci. 154, L225,1985. 5 C. S. Fadley. h o g . Sutf Sci. 16,275, 1984; C. S. Fadley. Pbysica Smpta. T17,39, 1987. In this and the following reference, the author gives a detailed overview of XPD and AED, including a quantitative description of the scattering theory behind the diffraction effect, instrumental considerations, and comparisons with related techniques like extended X-ray absorption fine structure (EXAFS) and low-energy electron diffraction (LEED).Reference 6, especially, has a comprehensiveprofile on recent XPD and AED experiments that have strongly contributed to the field. 6 C. S. Fadley. in Synchrotron Radiation Researcb:Advances in &$ace Science (R Z . Bachrach, ed.) Plenum Press, New York, 1989. 7 D. G. Frank, N. Batina, T. Golden, F. Lu, and A. T Hubbard. Science. 247,182,1990. 8 R S. Saiki, G. S. Herman, M. Yamada, J. Osterwalder, and C. S. Fadley. Pbys. Rev.Lett. 63,283, 1989. 9 W. F. EgelhofX Pbys.Rev. B30,1052,1984; E. L. Bullock and C. S. Fadley. Pbys.Rev. B31, 1212 (1985). i o S. C. Chambers, H. W. Chen, I. M. Vitomirov, S. B. Anderson, and J. H. Weaver. Pbys.Rev. B33,8810, 1986; B. Hermsmeier, R F. C. Farrow, C. H. Lee, E. E. Marinero, C. J. Lin, R. F. Marks, and C. J. Chien. ]. M a p . Mat. 1990, in press. 11 G. R Harp, D. K. Saldin, and B. E Tonner. Pbys.Rev.B42,9139, 1990. 12 B. Hermsmeier, J. Osterwalder, D. J. Friedman, B. Sinkovic, T. Tm, and C. S. Fadley. Pbys. Rev. B42,11895, 1990. 13 U. Breuer, 0. & a d , and H. E Bond.]. Kzc. Sci. Zcbnol. A8,2489, 1989; D. J. Friedman, T. Tran, and C. S. Fadley. The Structure ofsafaces (S. Y. Tong, M. A. Van Hove, X Xide, and K. Takayanagi, eds.) SpringerVerlag, Berlin, in press. 14 A. Stuck, J. Osterwalder, T.Greber, S. Hiifner, and L. Schlapbach. Pbys. Rev. Lett. 65,3029,1990. 4

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4.5

LEED Low-Energy Electron Diffraction MAX G . LAGALLY

Contents

.

Introduction Basic Principles Diffraction Measurements Examples Limitations of LEED Conclusions

Introduction

Low-energy electron diffraction (LEED) is a technique for investigating the crystallography of surfaces and overlayers or films adsorbed on surfaces. LEED is generally performed with electron energies of 10-1 000 eV. The limited penetration of electrons in this energy range provides the sensitivity to the surface. Diffraction of electrons occurs because of the periodic arrangement of atoms in the surface. This periodic arrangement can be conceptualized as parallel rows of atoms analogous to grating lines in a diffraction grating. Thus, diffraction in LEED occurs from rows of atoms, just as in its three-dimensional (3D) counterpart, X-ray diffraction, which can be considered as occurring from plan&. The diffracted beams emanate from the crystal surface in directions satisfying interference conditions from these rows of atoms. The diffraction pattern and the intensity distribution in the diffracted beams can provide information on the positions of atoms in the surfice and on the existence of various kinds of crystallographic disorder in the periodic arrangement of surface atoms.'-7 In its most elementary form, LEED can be used to test for the existence of overlayer phases having a two-dimensional (2D) crystal

252


Chapter 4

structure different from the surface on which they are adsorbed and to test whether a surface phase is ordered or disordered. Although LEED is the best-known and most widely used surface crystallographic technique, other diffraction techniques can provide information on the surface structure. Because of its convenient configuration, reflection high-energy electron diffraction (WEED) is used widely in.the epitaxial growth of films. X-ray diffraction can also be used for surface crystallography under appropriate conditions. Thermal-energy atom diffraction (TEAS) is a powerful technique for looking at disorder on surfaces because of the high sensitivity of low-energy atoms to isolated surface atoms. Surface extended absorption fine structure (SEXAFS) and photoelectron (XPD) diffraction are specialized techniques that also can give structural information on surfaces. WEED, XRD, XPD, and SEXAFS are covered in separate articles. Basic Principles Surface Crysta//ographyVocabulary

If an imaginary plane is drawn somewhere through a perfectly periodic 3D crystal, and the two halves of the crystal are separated along this plane, ideal surfaces are formed. If the imaginary plane corresponds to an (hkf) plane in the bulk crystal, the surface is defined as an (hkf)surface, using the usual Miller indices. The periodic arrangement of atoms in the surface can be viewed as a 2D lattice; that is, every point in this arrangement can be reached by a translation vector. The smallest translation vectors define the unit mesh, the 2D analog of the unit cell. Primitive unit meshes contain one lattice point per mesh; nonprimitive meshes, more than one lattice point per mesh. Figure l a shows the five 2D Bravais nets. The unit mesh vectors are conventionally defined, as shown, with the angle between the unit vectors 290°, a denoting the shorter unit vector, and b (aligned horizontally) the longer one. A 2D lattice may also have a basis. The lattice is defined by those points that can be reached by a translation vector. The basis is the conformation of atoms around each of these points. The arrangement of lattice points in a 2D lattice can be visualized as sets of parallel rows. The orientation of these rows can be defined by 2D Miller indices (hk see Figure lb). Inter-row distances can be expressed in terms of 2D Miller indices, analogous to the notation for 3D crystals. In the discussion so far, an ideal termination of the bulk crystal has been assumed at the surface; that is, the positions of atoms in the surface have been assumed to be the same as what they would have been in the bulk before the surface was created. This may not be true. Reconstruction, a rearrangement of atoms in the surfice and near-surface layers, occurs frequently. It is caused by an attempt of the surface to lower its free energy by eliminating broken bonds. The atomic layers par-

4.5

LEED

253

Square

Hexagonal

a

Figure 1

Unit meshes and 2D Miller indices. (a) Examples of 2D Bravais nets; (b) exampies of families of rows with Miller indices referenced to the unit mesh vectors. (11)and (31)families of rows are shown.

ticipating in this reconstruction can be considered as a different phase having a different periodic arrangement of atoms. Even when no reconstruction occurs, adsorption of a foreign species onto the substrate surfice from the ambient gas, from a deliberately created beam of atoms, or as a consequence of segregation of an impurity out of the bulk creates a surface phase termed an overlayer. An overlayer can be a fraction of a single atomic layer or several atom layers forming a thin film. The overlayer phase has its own crystal structure, which may or may not be related in a simple manner to that of the substrate surfice. Examples are shown in Figure 2. An overlayer may be commensurate or incommensuratewith the substrate. Commensurate overlayers have unit meshes that are related by simple rational numbers to those of the substrates on which they are adsorbed. Incommensurate layers do not. The unit mesh of overlayers is defined as a multiple of that unit mesh of the substrate surface that would be produced by the ideal termination of the bulk lata crystal have square- and parallelotice. Thus, (100) and (111) surfaces of an f gram-shaped primitive unit meshes, respectively (Figure la), both of which would be considered (1 x 1) unit meshes. Commensurate overlayer unit meshes are then p(m x n) or c(m x n) (Figures2a and 2b, respectively), where p and c refer to primitive and centered overlayer unit meshes relative to the primitive substrate unit mesh, and m and n are constants. An example of a complete unit mesh description of an overlayer on a particular substrate is W(110) p(2 x 1) 0,which describes an oxygen overlayer adsorbed on the (110) surfice of tungsten having a unit mesh twice that of the primitive W(110) unit mesh in the a direction and the same as the W(110) unit mesh in the b direction. 254


Chapter 4

b

b

aP

a

b

C

d

Examples of overlayer structures with appropriate notation. (a) fcc (100) ~ ( 2 x 2 ) (b) ; fcc (100)~ ( 2 x 2 ) (c) ; fcc (111) p(& X &)R3Oo; (d) bcc (110 I p(2 x 1) and bcc (110)p(1 X 2). The two orientationsfor the unit mesh in (d) are both possible because of the symmetry of the substrate; they have the same free energy and are called degenerate.

Figure2

The overlayer unit mesh is sometimes rotated relative to that of the substrate (Figure 2c). An example of the notation for such a structure is Ni (111) ( & x &) R30" 0,which describes an oxygen overlayer on Ni (111) whose unit mesh is & times as large as the substrate mesh in both unit mesh directions and is rotated by 30" relative to the substrate mesh. The symmetry of the substrate is often low, permitting the formation of energetically equivalent structures rotated relative to each other or translated by some specific amount, as shown in Figure 2d. Domains having symmetry related structure are called rotational or translational antipbase domains and are degenerate; that is, they have the same free energy, and there is no preference for the formation of one over the other. Overlayers with ( mx n) meshes generally also form (nx m) meshes; for example, p(2 x 1) + p(1 x 2) and c(2 x 4)+ c(4 x 2) (Figure 2d). Most low-symmetry surfices allow degenerate overlayer or reconstruction domains. Dmaction From Surfaces

Diffraction from surfaces can be viewed most simply as the scattering of waves from h i l i e s of lattice rows that connect scattering centers lying in a single plane (Figure 3). If a wave with wavelength h is permitted to fillat an angle of incidence 80 onto a h i l y of rows separated from each other by a distance du, the 2D h u e condition can be calculated for constructive interference between incoming and outgoing waves by considering the difference in paths traveled by the rays striking two adjacent rows of atoms:

mh = dhA(sinemhmk-sine,) where the difference in paths traveled by two adjacent rays is m wavelengths. The wavelength h of electrons (in A) is related to their energy E(given in eV) by 4.5

LEED

255

a

Figure3

b

Diffraction from a family of rows spaced 4kapart and its dependence on wavelength. Each solid circle represents a row of atoms in the plane of the paper. Rays with wavelength h fall on this family of rows at an angle 8,. Interference maxima occur at angles 0, that satisfy Equation (2). Only the firstorder reflection is shown. (a) Longer wavelength; (b) shorter wavelength.

For a fixed incident-beam energy, that is, a fmed wavelength, and a fixed angle of incidence, each family of (hk) rows diffracts radiation at the appropriate exit angle, 0,. If a fluorescent screen or other detector is positioned to intercept these scattered beams, a diffraction pattern having the symmetry of the surfice or overlayer unit mesh will be observed (Figure 4). At the center of the pattern will be the (00) beam, which has a path difference of zero and is therefore not sensitive to d k , that is, to the relative lateral positions of the surfice atoms. Around it w ill be the firstorder diffracted beams ( m= 1) from each family of possible rows that can be drawn through the surface atoms, for example, (lo), (Ol), (1 l), (21), and so on. In addition, there will be higher-order reflections ( m1 1) at larger angles. For example, the (20) and (30) reflections will f i U on the extension of a line connecting the (00) and (10) beams; the (00), (ll), (22), (33), etc., beams will be collinear, and so fbrth. Diffracted beams are indexed with the (hk) notation of the families of rows from which they are diffracted. Thus, the (10) beam is scattered from (10) rows in the direction perpendicular to these rows. The distance of the (hk) reflection from the mirror reflection, the (00)beam, is inversely related to the (hk) inter-row distance dh; see Equation (1). If the energy of the incident beam or the angle of incidence is changed, the diffraction angles eh adjust to continue to satisfy the h u e conditions. For example, if the energy is increased, the entire pattern will appear to shrink around the (00) beam because becomes smaller. Any periodicity in the su&e or overlayer different from that of the clean unreconstructed surface produces additional diffracted beams. A unit mesh larger than 256


...

Chapter 4

Second-order

Figure 4

Interference pattern created when regularly spaced atoms scatter an incident plane wave. A spherical wave emanates from each atom; diffracted beams form at the directions of constructive interference betweenthese waves. The mirror reflection-the (00) beam-and the first- and second-order diffracted beams are shown.

that of the substrate surface yields diffracted beams that are nearer each other; i.e., if d b is larger, 6fi is smaller; see Equation (1). The diffraction conditions can be depicted most easily using a reciprocal-lattice and Ewald construction (Figure 5).4 The Ewald sphere provides a schematic description of the conservation of energy; that is, since diffraction involves elastic scattering, the incident and exiting beams have the same energy. Its radius is inversely proportional to the wavelength X.The reciprocal lattice for a single layer of atoms consisting of families of rows is a set of rods (hk)normal to the crystal surkce having spacing 2Wdhk The intersection of the Ewald sphere and the reciprocal-lattice rods is a graphical solution of the h u e equation, Equation (l), and therefore yields the diffraction pattern. As the energy of the incident beam or the angle of incidence is varied, the radius of the Ewald sphere or its orientation relative to the rods changes, consequently also changing the points of intersection with the rods. The directions of the outgoing vectors define the directions of the diffracted beams. The discussion of diffraction so far has made no reference to the size of the 2D “grating.” It has been assumed that the grating is infinite. In analogwith optical or X-ray diffraction, finite sizes of the ordered regions on the surface (finite-sized gratings) broaden the diffracted beams. From an analysis of the diffracted-beam shapes, the types of structural disorder in the s u h c e region can be identified and quantified.7-9 Surface sensitivity in LEED is provided by the limited mean free path for inelastic scattering of slow electrons. This mean free path is the distance traveled by an electron in the solid before it collides inelastically, loses energy, and thus becomes 4.5

LEED

257

e-

1

b

a

Figure 5

Reciprocal-lattice and Ewald construction corresponding t o LEED and comparison t o real-spacepicture. (a) Real-space schematic diagram of diffraction from a surface. The electron beam is incident on the sample along the direc0, (00). tion given by e-. The five dfiacted beams represent the (hk), and (2h 2k) beams from a family of rows (hk) with spacing d,,k. (b) The corresponding cut through the reciprocal-lattice and the Ewald construction. The reciprocal-lattice rods are normal t o the crystal surface, given by the nearly horizontal line. They represent the same set of beams as in (a). The Ewald sphere always intersects the origin ofthe reciprocal lattice at the point of incidence of the incoming ray. Directionsof the dfiacted beams depend on the radius (2& of the Ewald sphere. Similarly, as the orientation of the crystal surface is changed in relm-on t o the incident beam, the reciprocal lattice rotates about its origin.

(ma,

lost for diffraction. The mean free path in the energy range of LEED is from 4 to 20 A. Because a layer spacing is of the order of 3 A, these slow electrons probe only a few atomic layers. The inelastic scattering of slow electrons is discussed further in other articles, e+, AES and X P S . Diffraction Measurements

Figure 6 shows a schematic diagram of a typical LEED system." The electron gun can produce a monoenergeticbeam of approximately 10-1000 eV, with beam sizes typically a fraction of a mm, but ranging in newer instruments down to -10 pm. The goniometer generally allows two sample motion- rocking about an axis in the plane of the crystal surface and rotation about an axis normal to the crystal surhce-as well as heating and sometimes cooling of the sample and temperature measurement capability. The detector consists usually of concentric grids and a fluorescent screen. The grids filter inelastically scattered electrons that also emanate 258


Chapter 4

Screen

Goniometer

Window

Electron gun Figure 6

LEED diffractometer.The vidicon camera can be interfacedwith a computerto record the deaction pattern displayed on the fluorescent screen. The Faraday collector may contain a channeltron electron multiplier for added sensitivity.

from the crystal surface. For accurate measurement of beam currents and beam profiles a Faraday cup detector can be used. The Faraday cup is biased to exclude inelastically scattered electrons. Modern detectors incorporate a channeltron electron multiplier or a resistive anode encoder in the detector, depending on the need for sensitivity or resolution.'' The simplest diffraction measurement is the determination of the surface or overlayer unit mesh size and shape. This can be performed by inspection of the diffraction pattern at any energy of the incident beam (see Figure 4). The determination is simplest if the electron beam is incident normal to the surfice, because the symmetry of the pattern is then preserved. The diffraction pattern determines only the size and shape of the unit mesh. The positions of atoms in the surfice cannot be determined from visual inspection of the diffraction pattern, but must be obtained from an analysis of the intensities of the diffracted beams. Generally, the intensity in a diffracted beam is measured as a function of the incident-beam energy at several diffraction geometries. These "intensity-versus-energy" curves are then compared to model calculations.' Diffraction is useful whenever there is a distinct phase relationship between scattering units. The greater the order, the better defined are the diffraction features. For example, the reciprocal lattice of a 3D crystal is a set of points, because three Laue conditions have to be exactly satisfied. The diffraction pattern is a set of sharp spots. If disorder is introduced into the structure, the spots broaden and weaken. Two-dimensional structures give diffraction "rods," because only two h u e conditions have to be satisfied. The d i h c t i o n pattern is again a set of sharp spots, because the Ewald sphere cuts these rods at precise places. Disorder in the plane broadens the rods and, hence, the diffraction spots in x and y. The existence of streaks, broad spots, and additional difhse intensity in the pattern is a common 4.5

LEED

259

occurrence indicating that the overlayer or surface structure is not perfect. The angular profile of diffracted beams provides information on disorder and finite-size effects in surfaces analogous to particle size broadening in X-ray diffraction of bulk materials. The measurement of S E D angular profiles as a function of diffraction parameters, such as energy, angles, and beam order, makes it possible to distinguish types of surfice disorder, including finite island sizes, strain, crystal mosaic, monatomic and multiatomic steps at the surface, and regular or irregular domain or To measure a profile, a detector with a narrow slit or a antiphase bo~ndaries.~-~ point aperture must be used to minimize the influence of instrumental broadening. lo Examples of Uses Surhce-sensitive diffraction require a high vacuum because surface contamination rates from ambient gases are rapid. Contaminated surfaces are generally disordered and generally do not provide diffraction patterns. Because the beam penetration is so small, even a fav layers of adsorbed gas or oxide can mask the underlying pattern. The ideal surfice preparation method is cleavage in vacuum. Cleavage exposes an internal i n t e h that has not been subjected to ambient atmospheric contamination. However, cleavage is limited to a few crystals and generallyto only one surface orientation of these crystals. Samplesnot cleaved in vacuum must be cut from a single crystal, polished, and oriented carefilly to the desired surface using a Laue diffraction camera. Standard polishing procedures are used. LEED can be used to determine the atomic structure of surfaces, surfice structural disorder, and to some extent, s h c e morphology, as well as changes in suucture with time, temperature, and externally controlled conditions like deposition or chemical reaction. Some examples are briefly discussed here: 1 Surfice order and cleanliness. The most common use of LEED is in association with other surhce analysis or surface research methods, to check surface structural order and cleanliness.A visual display of the diffraction pattern is used. An expected pattern, with sharp dificted beams, is an indication that the s d c e is clean and well ordered. z Surfice aromic structure. The integrated intensity of several diffracted beams is measured as a function of electron beam energy for different angles of incidence. The measurements are fitted with a model calculation that indudes multiple scattering. The atomic coordinates of the surfice atoms are extracted! (See also the article on EXAFS.) 3 Step density. For su&ces containing steps, both the step height and step density can be determined by measuring the angular profile of diffracted beams (the shape of the beam in angle) at specific conditions of energy and angle. This measurement can be static or dynamic, the latter if the surface is evolving with 260


...

Chapter 4

cime, as a function of temperature or adatom coverage. The analysis is straightforward in terms of kinematic t h e ~ r y . ~Similar - ~ measurements can identify surface strain, the existence of structural domains or islands, and crystal m ~ s a i c . ~ (For crystal mosaic see also the article on XRD.) Phase transitions in overlayers or surfaces. The structure of surface layers may undergo a transition with temperature or coverage. Observation of changes in the diffraction pattern gives a qualitative analysis of a phase transition. Measurement of the intensiey and the shape of the profile gives a quantitative analysis of phase boundaries and the influence of finite sizes on the tran~ition.~ Dynamics of phase transitions, ordering, disordering, and growth. Measurement of the time evolution of the diffracted intensity and the shape of diffracted beams gives information on the kinetic mechanisms of ordering and disordering, growth laws, and finite-size effect^.^ Transitions from layered to cluster growth, as well as transitions from amorphous to crystalline phases, can be investigated. Activation energies for diffusion and domain boundary motion can, in principle, be extracted. Thermal properties of overlayer atoms. Measurement of the intensity of any diffracted beam with temperature and its angular profile can be interpreted in terms of a surface-atom DebyeWaller factor and phonon scattering? Mean-square vibrational amplitudes of surface atoms can be extracted. The measurement must be made away from the parameter space at which phase transitions occur. Chemical reactions of surfaces. Diffraction can be used qualitatively to identify different surface phases resulting from adsorption and chemical reaction at surf a c e ~Reaction .~ rates can be investigated by following the evolution of diffracted beam intensities. Grain size in textured films. For films having a preferred growth direction+.g., (111)-LEED can be used to determine the preferred direction and the grain size parallel to the surface. The preferred direction is obtained from the symmetry of the diffraction patterns, while the grain size is obtained from the shape in angle of diffracted beams. Limitationsof LEED

Surface-sensitivediffraction is, for the most part, restricted to analysis of surfaces of single crystals and overlayers and films on such surfaces. If a polycrystalline sample is illuminated using a beam of low-energy electrons, each crystallite surface exposed will create its own diffraction pattern, all of which will be superimposed on the fluorescent screen detector. If more than a few orientations are illuminated by the beam, the panern becomes too complicated to analyze. However, if the size of the 4.5

LEED

261

LEED beam is reduced to less than the sample grain size, individual grains in a polycrystal can be investigated. It is difficult to make low-energy electron beams this small, but in some materials large grain sizes can be achieved in polycrystals. Because electrons are charged, only materialshaving reasonable conductivitycan be investigated. Insulators and ceramics pose difficulties because charge accumulates on the sample and eventually prevents the incident beam from striking the surfice. Most insulators can be investigated with special techniques, such as providing surface conduction paths near the analysis area or using very thin samples mounted on a conductingplate. The use of LEED for quantitative determinationsof atomic positions in surfices is complicated by the multiple scattering of the electrons, a process that is highly sensitive to scattering geometry. An accurate goniometer and detection scheme are required to measure intensities for which the scatterihg geometry is precisely known.A detector with high sensitivityis required to measure the frequently small intensities in this type of measurement, but high resolution is not necessary.Massive calculations that include multiple scattering are necessary to fit data to extract atomic positions. Measurements of surface disorder require a high resolving power (the ability to distinguish two dose-lying points in the diffraction pattern). Quantitative measurements of surface disorder are limited in the following manner: the worse the resolving power, the smaller the maximum scale ofsurface disorder that can be detected. For example, if the maximum resolvable distance of the diffractometer is 100A, then a surface that has steps spaced more than 100 A apart will look perfect to the instrument. The theoretical analysis of disorder is much simpler than that for atomic positions. Conclusions

LEED is the most powerhl, most widely used, and most developed technique for the investigation of periodic surface structures. It is a standard tool in the surfice analysis of single-crystal SUA-. It is used very commonly as a method to check surfice order. The evolution of the technique is toward greater use to investigate surfice disorder. Progress in atomic-structure determination is focused on improving calculations for complex molecular surfice structures. Preparation of this article was supported by the National Science Foundation, Solid State Chemistry Program. Relnted Artides in the Encyclopedin

M E E D , XRD, XPD, and EXAFS

262


Chapter 4

References 1 2

3

4

5

6

7

8

9

10

4.5

J. J. Lander. hog. Solid State Cbem. 2, 26, 1965. E J. Estrup. In Modern Difiaction and Imaging Techniquesin Matenah Science. ( S . Amelinch, R Gevers, G. Remaut, and J. Van Landuyt, Eds.) North-Holland, 1970;and l? J. Estrup and E. G. McRae. Surf Sci. 25, 1, 1771. G. A. Somorjai and H. H. Farrell. Mu.Cbem. Pbys. 20,215, 1972. Contains listings of overlayer phases that have been observed to that date. M. B. Webb and M. G. Lagally. SolidState Pbys. 28, 301, 1973. A detailed discussion at a more advanced level. G. Ertl and J. Kiippers. Low-EnergyElectrons andsurface Cbemistry.Verlag Chemie, Weinheim, 1974, Chps. 9 and 10. An introductory treatment of diffraction. J. B. Pendry. Low-Energy Electron Dzfimtion. Academic Press, New York, 1974.Theoretical treatment, principally on surface atomic structure determination. M. G. Lagally. In Metbod of ExperimentalPbysicdurfaces. (R L. Park and M. G. Lagally, Eds.) Academic Press, New York, 1985, Vol. 22, Chp. 5. M. Henzler. Top. Cum Pbys. 4, 117, 1977. Basic information on use of LEED to analyze steps and step disorder on surkes. M. G. Lagally. In Reflection Higb-Energy Electron Difimtion andR@ection Electron ImagingofSurfaces.(E K. Larsen and l? J. Dobson, Eds.) Plenum, New York, 1989. M. G. Lagally and J. A. Martin. Rev Sci. Insk 54, 1273,1983.A review of LEED instrumentation.

LEED

263

4.6

RHEED Reflection High-Energy Electron Diffraction D O N A L D E. SAVAGE

Contents

Introduction Basic Principles Applications Conclusions

Introduction

Reflection High-Energy Electron Diffraction (WEED) is a technique for probing the surhce structures of solids in ultrahigh vacuum (UHV). Since it is a difhctionbased technique, it is sensitive to order in solids and is ideally suited for the study of clean, well-ordered single-crystal surfices. In special cases it can be used to study clean polycrystalline samples as well. It gives essentially no information on the structure of amorphous surfaces, which makes it unsuitable for use on sputtercleaned samples or in conjunction with sputter depth profiling, unless the sample can be recrystallized by annealing. The area of a surface sampled in W E E D is determined by the primary beam size at the specimen. The typical electron gun used for W E E D focuses the electrons to a spot on the order of 0.2 mm in diameter. The diffraction pattern should be interpreted as arising from a spatial average over an area whose width is the beam diameter and whose length is the beam diameter divided by the sine of the incidence angle. Of course, high-energy electrons can be focused to a much smaller spot, on the order of several k The detection of a W E E D feature as an electron beam is rastered along the surfice is the basis for Scanning Reflection Electron Microcopy (SREM). The surfice sensitivity of M E E D comes from the strong interaction between electrons and matter. Electrons with W E E D energies and a 264


Chapter 4

grazing angle of incidence will scatter elastically from only the first few atomic layers. One disadvantage of using electrons is that the sample must be sufficiently conducting so as not to build up charge and deflect the primary beam. Also, electronsensitive materials can be damaged during a measurement. When used to examine a crystal surface, W E E D gives information on the surface crystal structure, the surface orientation, and the degree of surfice roughness. Evidence for surface reconstruction (the rearrangement of surface atoms to minimize the surface energy) is obtained directly. In addition, when RHEED is used to study film growth on crystalline surfaces, it gives information on the deposited material's growth mode (i.e., whether it grows layer-by-layer or as three-dimensional (3D) crystallites), the crystal structure, the film's orientation with respect to the substrate, and the growth rate for films that grow layer-by-layer. RHEED is particularly useful in studying structure changes dynamically, i.e., as hnction of temperature or time. Combined with its open geometry (the area normal to the surface will have a direct line of sight to deposition sources) this feature makes it one of the most commonly used techniques for monitoring structural changes during molecular beam epitaxy (MBE). Other diffraction techniques whose principles are similar to W E E D include Low-Energy Electron Diffraction (LEED), Thermal-Energy Atom Scattering (TEAS), and X-ray Diffraction. Of these, LEED is the most similar to RHEED, differing mainly by its normal-incidence scattering geometry. Atom scattering differs from RHEED in that it is more surface sensitive than electron difiaction because atoms scatter off the outer electronic shells of atoms in the surfice. This makes TEAS very sensitive to defects like atomic steps, but corrugations due to the regular positions of atoms in the surface are difficult to observe. X-ray diffraction is the classic technique for measuring bulk crystal structure. With the use of highbrightness X-ray sources, the surface structure also can be determined by grazing incidence methods. Basic Principles

The underlying principle of M E E D is that particles of matter have a wave character. This idea was postulated by de Broglie in (1924). He argued that since photons behave as particles, then partides should exhibit wavelike behavior as well. He predicted that a particle's wavelength is Planck's constant h divided by its momentum. The postulate was confirmed by Davisson and Germer's experiments in 1928, which demonstrated the difiaction of low-energy electrons from Ni.2 For nonrelativistic electrons, the wavelength (in A) can be written

4.6

RHEED

265

\

Figure 1

Plane wave scattering from two consecuh've lines of a onedimensional diffraction grating. The wave scatters in-phase when the path difference (the difference in length of the two dotted sections] equals an integral number of wavelengths.

where E is the energy of the electron (in electron volts). For a primary beam energy of 5-50 keV this relationship gives h ranging from 0.17 to 0.055 A. Most of the features of a M E E D pattern can be understood qualitativelywith the use of kinematic scattering theory, i.e., by considering the single scattering of plane waves off a collection of objects (in this case atoms). In the kinematic limit, the scattering of electrons from a single-crystal surface can be treated in the same way as the scattering of monochromaticphotons from a two-dimensional (2D) diffraction grating. Given the wavelength and angle of incidence of the source radiation, the angles at which diffracted beams are scattered will depend on the grating line spacing (i.e., the atomic row spacing). Consider a plane wave of wavelength h incident on a onedimensional (1D) grating with line spacing d as shown in Figure 1. The diffraction maiima occur when successive rays scatter in-phase, i.e., when their path difference is an integral number of wavelengths. The grating equation

nh =

d(cosex-

(2)

Cd+,)

defines conditions where the maxima occur. Using Equation (2) one can show that to see higher order diffraction maxima when scattering from atomic rows, their relatively close spacing requires that the source wavelength be short. The analogy of a crystal surface as a diffraction grating also suggests how surface defkcts can be probed. Recall that for a diffraction grating the width of a diffiacced peak will decrease as the number of lines in the grating is in~reased.~ This observation can be used in interpreting the shape of W E E D spots. Defects on a crystal s u r f i c e can limit the number of atomic rows that scatter coherently, thereby broadening W E E D features. 266


Chapter 4

In a diffraction experiment one observes the location and shapes of the diffracted beams (the diffraction pattern), which can be related to the real-space structure using kinematic diffraction theory! Here, the theory is summarized as a set of rules relating the symmetry and the separation of diffracted beams to the symmetry and separation of the scatterers. The location of a diffracted beam can be defined by specifylng the magnitudes and the directions of the incoming and outgoing waves. This can be written in a shorthand notation using the momentum transfer vector S,where S = I?,ut - Fn. The vector K is the wave propagation vector and has units of inverse length. Its magnitude is 2n/h, and it points along the direction of wave propagation. The vector S can be thought of as the change in momentum of a wave upon scattering. The periodicity of the scattererswill constrain the values of S where in-phase scattering occurs. The dimensionality of the diffraction problem will have a strong effect on how the diffraction pattern appears. For example in a 1D problem, e.%.,diffraction from a single line of atoms spaced dapart, only the component of S in the direction along the line is constrained. For a 2D problem, e.g., the one encountered in RHEED, two components of S in the plane of the surface are constrained. For a 3D problem, e.g., X-ray scattering from a bulk crystal, three components of S are constrained. For a given structure, the values of S at which in-phase scattering occurs can be plotted; these values make up the reciprocal lattice. The separation of the diffraction maxima is inversely proportional to the separation of the scatterers. In one dimension, the reciprocal lattice is a series of planes, perpendicular to the line of scatterers, spaced 2n/dapart. In two dimensions, the lattice is a 2D array of infinite rods perpendicular to the 2D plane. The rod spacings are equal to 2n/(atomic row spacings). In three dimensions, the lattice is a 3D lattice of points whose separation is inversely related to the separation of crystal planes. Kinematic Dieaction from a 20 Plane

Electrons having energies and incident angles typical of RHEED can be treated as nearly nonpenetrating. As a result, atoms in the outermost plane are responsible for most of the scattering, and the resulting reciprocal lattice will be an array of rods perpendicular to the surface plane. The symmetry and spacing of the 2D reciprocal-lattice mesh (the view looking down upon the array of reciprocal-lattice rods) will depend on the symmetry and spacing of atomic rows in the surfice. Consider a crystal plane with lattice points located on the parallelogram shown in Figure 2a. The corresponding reciprocal mesh is shown in Figure 2b. One can construct this reciprocal-mesh by defining two nonorthogonal mesh vectors A* and B*, whose lengths are equal to 2n divided by the separation between two adjacent atomic rows, and whose directions lie in the plane of the surfice and are perpendicular to those rows. The (h, k) reciprocal-lattice rod is located by the vector %= /A* + kB*, where hand k are integers. Note 4.6

RHEED

267

...

b

\

1

I I

I

I

B*

27T

b(sinq)

I I I I I I I I I

Real-Space Figure 2

Net

Reciprocal-Space

Mesh

view lookingdown on the real-spacemesh (a) and the corresponding view of the reciprocal-space mesh (b)for a crystal plane with a nonrectangular lattice. The reciprocal-space mesh resembles the realapace mesh, but rotated 90". Notethat the magnitudeof the reciprocallatticevectors is inversely related to the spacing of atomic rows.

that the symmetry of the surfice reciprocal-mesh is the same as the symmetry of the surfice lattice rotated by 90". This result will always hold. A di&ction pattern will be influenced strongly by the direction of the primary beam relative to the surfice. The grazing angles used in M E E D can make the interpretation of the pattern difficult. For a specific choice of &,i.e., primary beam energy and direction, the direction in which beams will be diffracted can be determined using the Ewald construction. This is a graphical construction that uses conservation of energy (for elastic scattering IKI is conserved) and momentum o(outmust lie on the reciprocal lattice). It is done by drawing Kin terminating at the origin of reciprocal-space. A sphere of radius I KI centered at the origin of qn gives the locus of all possible scattered waves that conserve energy. The intersection of the sphere with the reciprocal-lattice locates the diffracted beams. Consider as an example the W E E D pattern from GaAs (110). Gallium Arsenide can be cleaved along (110) planes; the resulting surfice is nearly perfect. In Figure 3a the surfice real-space lattice and the reciprocal-spacemesh of GaAs (110) are shown. For GaAs (110), the surface real-space net is rectangular and has a 2-atom basis; only the lattice points (not the atom locations) are depicted. Figure 3b shows a portion of the Ewald construction for a primary beam incident along an azimuth parallel to the real-space lattice rows (spaced 161 apart), i.e., in the a direction, also called an [OOl] direction, and 20° from grazing. Note that the intersections of rows of reciprocal lattice rods with the Ewald sphere lie on circles similar to circles of constant latitude on a globe. If the di&cted beams are pro268


Chapter 4

a

I

I

I

I

I

+%--I

[0.0,11

Real-Space N e t

Figure 3

4.6

RHEED

Reciprocal-Space

Mesh

(a) Real-space lattice and reciprocal-space mesh for the GaAs (110) plane. (b) Ewald constructionfor this surface with a primary beam incident along the a direction (the[OOl] azimuth) and elevated 20" from grazing. 269

d

Figure 3

(c) Photograph of a RHEED pattern from cleaved GaAs(l10) obtained using a 10 KeV beam incident along the 10011 azimuth at an angle of 2.4O-the spots lie along arcs (the inner is the 0th Laue circle, and the outer is the first Laue circle). The pattern is indexed in the drawing below the photo. (d)Photograph of the RHEED pattern after the sample has been splitter etched with 2-KeV Argon ions and subsequently annealed at 500" C for 10 minutes. The pattern is streaky, indicating the presence of atomic steps on the surface.

jected forward onto a phosphor screen, they will appear as spots lying on semicircles in the same way that lines of constant latitude will inscribe circles if projected onto a screen located at the north pole of a globe. These rings are called Laue circles and are labeled 0, 1, etc., starting with the innermost. For this example, the 0th Laue zone contains the set of (0, k) reciprocal lattice rods. Note that if the crystal is rotated, thereby changing the orientation of primary beam azimuth so that it is no longer parallel to a set of rows, the projected diffracted beams will no longer lie on semicircles, and the diffraction pattern will appear skewed. Figure 3c shows a photograph of a RHEED pattern from a cleaved GaAs (1 10) surface obtained with a IO-keV primary beam directed along an [OOl] azimuth. The diffracted maxima appear as spots lying along circles, as predicted, and are indexed in the view to the right. The diffracted spots of the 0th and 1st Laue circles are shown. One can tell from this view that the symmetry of the GaAs (110) surface net is orthogonal because the spots in the 1st Laue circle lie directly above those in the 0th. If the surface net had been a parallelogram, as in Figure 2b, the spots in the 1st Laue circle would be displaced. The spacing of the (0, k) rows is obtained from 270


Chapter.4

the spacing of the spots on the Laue circle, and equals 27t/ 6. The spacing of the (h, 0) rows (the lattice rows perpendicular to the [OOl] direction) can be obtained from the spacing between h u e circles, and equals 25~/a.In practice, it is more common to rotate the sample until the primary beam is parallel to the (h, 0) atomic rows (for an orthogonal net, rotation by 90") and to obtain the row spacing from the lateral spacing of the spots in that pattern. Some other the features to note in the pattern are the so-called transmitted beam, the shadow edge, and the specular reflection. The transmitted beam is the bright spot at the bottom of the photo. It arises when part of the incident beam misses the sample and strikes the phosphor screen directly. This is done intentionally by moving the sample partly out of the beam. The spot, also called the (000) beam, is a useful reference point because it locates the origin of reciprocal space. The spot directly above the transmitted spot is the specular reflection. Halfway between them is the shadow edge, below which no scattered electrons can reach the phosphor screen. Figure 3d is a photograph of a cleaved GaAs (1 10) sample taken under conditions similar to Figure 3c but after the sample had been sputter etched with 2-keV Ar ions and subsequently annealed at 500" C for 10 minutes. Because the treatment resulted in a less intense diffraction pattern, the exposure time for this photograph was increased. This W E E D pattern shows streaks instead of spots, obscuring the location of the specular reflection. The separation of the streaks equals 2n;/ 6, as in the freshly cleaved surhce, but the angular separation from circle to circle is no longer clear. The streaks arise because atomic steps are created during the sample treatment that limit the long-range order on the surface and broaden the reciprocallattice rods. Their intersections with the Ewald sphere produce the elongated streaks. Defects in the surface also give rise to a diffuse background intensity, making the shadow edge more visible. The effect of disorder on the diffraction pattern will be discussed more in the section on nonideal surfaces. Applications Surface Reconstruction

The size and symmetry of the surface lattice, which can differ from the bulk termination, is determined directly from the symmetry and spacing of beams in the diffraction pattern.' Because surface atoms have a smaller coordination number (i.e., fewer nearest neighbors) than bulk atoms they may move or rebond with their neighbors to lower the surface energy. If this occurs, the surfice real-space lattice will become larger; i.e., one must move a longer distance on the surface before repeating the structure. As a result additional reflections, called superlattice reflections, appear at fractional spacings between integral-order reflections. One example of this is the (7 x 7) reconstruction of Si (111). The real-space lattice for this reconstructed surface is 7 times larger than the bulk termination. If the sample is well prepared, 6 fractional-order spots are observed between the integral-order spots on 4.6

RHEED

27 1

a h u e circle and, more importantly, 6 fractional-order h u e circles are ~bserved.~ In MBE, one of the uses of RHEED is to monitor the surface reconstruction during deposition to obtain optimal growth conditions. An example is in the growth of GaAs on GaAs (loo), where different reconstructions are observed depending on the surface stoichiometry. Deposition parameters can be varied to obtain the desired reconstruction. Nonideal Surfaces

All real surfaceswill contain defects of some kind. A crystalline surface must at the very least contain vacancies. In addition, atomic steps, facets, strain, and crystalline subgrain boundaries all can be present, and each will limit the long-range order on the surface. In practice, it is quite difficult to prepare an atomically flat surfice. Because defects limit the order on a surface, they will alter the diffraction pattern, primarily by broadening diffracted beams.* Methods have been developed, mostly in the LEED literature, to analyze the shape of diffracted beams to gain information on step distributions on surfaces. These methods apply equally well to RHEED. A statement that one will find in the MBE literature is that a smooth surface will give a “streaky”RHEED pattern while a rough surficewill give a “spotty”one. This is in apparent contradiction to our picture of an ideal RHEED pattern. These statements can be reconciled if the influence of defects is included. A spotty RHEED pattern that appears difkrent from spots along a Laue circle will result from transmission of the primary beam through asperities rising above the plane of a rough surface. This type of pattern is described below. A flat surface, one without asperities but having a high density of defects that limit order within a plane, will give rise to a streaky RHEED pattern. If a smooth and flat surface is desired, a streaky pattern is preferable to a transmission pattern, but a surface giving a streaky pattern is not ideally flat or smooth. Limiting long-range order within the plane of the surface will broaden the reciprocal lattice rods uniformly. For a plane with dimensions L x I, the width of a rod will be approximatelyequal to 2WL. The streaking is caused by the grazing geometry of RHEED. The Ewald sphere will cut a rod at an angle such that the length is approximately 1/(sine,) times the width, where 8, is the angle the exit beam makes with the surfice. For 8, 3”, this is a factor of 20. What gives rise to streaks in a RHEED pattern from a real surface? For integralorder beams, the explanation is atomic steps. Atomic steps will be present on nearly all crystalline surfaces. At the very least a step density sufficient to account for any misorientation of the sample from perfectly flat must be included. Diffraction is sensitive to atomic steps.9They will show up in the RHEED pattern as streaking or as splitting of the diffracted beam at certain diffraction conditions that depend on the path difference of a wave scattered from atomic planes displaced by an atomic step height. If the path difference is an odd multiple of h / 2 , the waves scattered

-

272


...

Chapter 4

from each plane will destructively interfere. For such a diffraction condition (called an out-of-phase condition) the surface will appear to be made up of finite sized patches and the reciprocal lattice rod will be broadened. In contrast, if the path length differs by an integral multiple of I, the surface will appear perfect to that wave and the no broadening is observed. This is called an in-phase condition. The width of a reciprocal-lattice rod at the out-of-phase condition is proportional to the step density on the surface. Another type of disorder that will broaden superlattice rods only, is the presence of antiphase domains. These domains occur because patches of reconstructed surface can nucleate in positions shifted from one another. Electrons will scatter incoherently from them, each domain acting as if it were a 2D crystal of finite size. This broadening will be independent of the position along the superlattice rod; i.e., there are no in-phase or out-of-phase positions for this kind of disorder. Transmission features will be present if the surface is sufficiently rough. These arise because electrons with RHEED energies can penetrate on the order of 100 through a solid before inelastically scattering. In W E E D , su&e sensitivity is obtained in part from the grazing geometry; i.e., electrons must travel a long distance before seeing planes deep in the bulk. For a rough surface, asperities rising above the surface will be struck at a less glancing angle and if the asperity is thin enough in the primary-beam direction, transmission will occur. Transmitted electrons will see the additional periodicity of the atomic planes below the surhce. As a result, a constraint on S in the direction perpendicular to the surface is added and the reciprocal lattice will be an array of points instead of rods. Because transmission can occur only through thin objects, the 3D reciprocal-lattice points from the asperities will be elongated in the direction of the primary beam. The Ewald sphere will intersect a number of the reciprocal-lattice points in the plane containing the 0th Laue circle (called the 0th Laue zone), giving rise to a pattern that will appear as a regular array of spots. It appears as a projection of the reciprocal lattice plane that is nearly perpendicular to the incident beam. The spacing of the spots is inversely related to the spacing of planes in the crystal. In addition to differences in their appearances, a practical way to differentiate between transmission and reflection features is by observing the diffraction pattern while changing the incident azimuth. The intensity of transmission features will change as the azimuth is changed, but their location will not. In contrast, reflection features (other than the (0, 0) beam) will move continuously either closer to or farther away from the shadow edge as the azimuth is changed, a result of the intersection of the Ewald sphere with a rod moving up or down as the azimuth is changed.

a

Film Growth

One of the main uses of WEED is to monitor crystal structure during film growth in ultrahigh vacuum. Its ability to distinguish between 2D and 3D structure gives 4.6

RHEED

273

direct evidence on the growth mode of a film. The onset of clustering is associated with the appearance of transmission features. In 2D film growth, changes in the surfice lattice periodicity (reconstruction) show up in the appearanceor disappearance of superlatticefeatures,” while roughness shows up as a change in the shape of the W E E D features. In addition, changes in the separation between W E E D k lattice constant, can be used streaks, which relate directly to changes in the s to follow strain relaxation. In films that grow 2D for many layers, “intensity oscillations” have been observed for certain growth conditions using WEED’* and LEED. Observation is made by monitoring the intensity of a diffracted beam as a function of time during growth. The period of an oscillation corresponds to the time it takes to deposit a monolayer. In practice, oscillationsare frequentlyused to calibrate depositionrates. Oscillations arise from periodic changes in the surfice structure as a deposited layer nucleates, grows, and then fds in the previously deposited layer. They can be understood qualitatively within the h e w o r k developed in the discussion of atomic steps. As a material is deposited, 2D islands nucleate, increasing the step density. This causes diffraction features to become smeared out, reducing their intensity at the location of the detector. As the surfice becomes covered, the intensity recovers. This process is repeated, with continued deposition causing the intensity to oscillate. Further evidence fbr t h i s interpretation has been obtained from the analysis of W E E D diffracted-beam profiles for the growth of GaAs on GaAs (loo), where the shape of d&cted beams has been modeled to extract step densities.12 While kinematic difiaction theory describes intensity oscillationsadequately in some cases,there are problems with it when it is used to analyze M E E D measurements. The period of the oscillations is correctly predicted, but not necessarily the phase. In spite of these complications, intensity oscillations are evidence for perik structure. odic changes in the s For deposited materials that are not perfectly lattice-matched to a substrate, strain will build up in the deposited layer until eventually it becomes energetically favorable for 3D clusters of the deposited material’s relaxed crystal structure to form. Some materials will not wet a substrate at all, and grow as 3D clusters at submonolayer coverages. W E E D can give information on cluster orientations, shapes, and sizes. Figure 4a is a photograph of a W E E D pattern obtained for 4 monolayers of In deposited on cleaved GaAs (1 10). The regular array of spots is clearly a transmission pattern and indicates that the deposited In has fbrmed 3D clusters. The pattern can be indexed with the symmetry and spacing of an Indium I:1101 reciprocal lattice plane, as shown in Figure 4b. Since all spots are accounted for, clusters present are oriented in the same direction with respect to the substrate. Different orientationswould show up as additional arrays of spots, and in the limit of a random orientation the pattern would appear as continuous rings centered about (O,O,O). Information on cluster shape can be extracted if the clusters have a 274


...

Chapter 4

0

Figure4

(0.0.0)

(a) Photograph of a RHEED pattern for 4 monolayers of In deposited on cleaved GaAs (110). The primary beam is incident along the GaAs 11101 azimuth. The regular array of spots are a result of transmission diffraction through indium 3D clusters. The streaks of intensity connecting the spots are from the reflection pattern off facets of the In clusters. (b) Spots in the photo can be indexed as the In 17101 reciprocal-lattice plane oriented 50 that the In (113) real-space plane is in contact with the substrate.

preferred orientation and are bounded by crystal facets. In Figure 5a the streaks connecting the spots are reflection features from In facet planes. (They are from (1 11)-and (001)-type facets.) Cluster size information is contained in the shape of the transmitted features and attempts have been made to extract quantitative values. l3 4.6

RHEED

275

Conclusions

W E E D is a powerid tool for studying the surface structure of crystalline samples in vacuum. Information on the surface symmetry, atomic-row spacing, and evidence of surface roughness are contained in the RHEED pattern. The appearance of the M E E D pattern can be understood qualitatively using simple kinematic scattering theory. When used in concert with MBE, a great deal of information on film growth can be obtained. The time evolution of the RHEED pattern during film growth or during postgrowth annealing is a subject of current interest. W E E D intensity oscillations have been observed to damp out over time during deposition. If growth is interrupted the M E E D pattern recovers. Initial attempts have been made to extract surface diffusion coefficients and activation energies by measuring the rate of such processes at different temperatures. RHEED intensities cannot be explained using the kinematic theory. Dynamical scattering models of M E E D intensities are being deve10ped.'~With them one will be able to obtain positions of the surface atoms within the surface unit cell. At this writing, such modeling has been done primarily for LEED. M E E D differs from LEED because of its grazing geometry and higher electron energies. It is the grazing geometry that allows RHEED to be used in concert with film growth. In addition, both reflected and forward scattered (transmitted) electrons can be observed with RHEED. The latter are not detected with LEED, making it less suited for the study of 3D roughness, i.e., clusters or very rough surfaces.


LEED and XRD References 1

2 3

4

5

L. de Broglie. Dissertation, Paris, 1924. C. J. Davisson and L. H. Germer. Pbys. Rm. 30,705,1927. For a basic treatment of interference and diffraction from a 1D grating, see E. Hecht. Optics. Addison-Wesley, Reading, 1987, Chapter 10. For a review on the formulation of kinematic difiaction theory with emphasis on the scattering of low-energy electrons, see M. G. Lagally and M. B. Webb. In: Solid State Pbysics. (H. Ehrenreich, E Seitz, and D. Turnbull, eds.) Academic, New York, 1973, Volume 28. For more derail on the geometrical relationship between the RHEED pattern and the surface crystal structure, see J. E. Mahan, K. M. Geib, G. Y. Robinson, and R G. Long. J Vu.Sci. Tecbnol. AS,3692,1990.

276


Chapter 4

For a review of how various&s reconstructions appear in a diffraction pattern, see M. A. Van Hove, W. H. Weinberg, and C. -M. Chan. LowE q Elcchon Dzj’kction. Springer, Berlin, 1986, Chapter 3. 7 S. Hasegawa, H. Daimtsn, and S. Ino. Surf: Sci. 187, 138,1987. 8 For a review of how defects manifest themselves in a LEED experiment, see M. Henzler. In: Ekctron Spcctroscopyfir Sufmc Ana+. (H. I. Ibach, ed.) Springer, Berlin, 1977. 9 For more information on kinematic treatment of diffraction from stepped In: wetsurhces, see M. G. Lagally, D. E. Savage, and M. C.T-des. tion High-Energy Ekctron Dimetion and RtfEection Electron Imaging of Sufmes. NATO AS1 Series ByPlenum, New York, 1988, Volume 188. i o Surface phase transformationsand surfice chemical reactions are fbllowed by studying the time evolution of superlatticebeams originating fiom monolayer or submonolayer films. See,for example, Chapters 8-10 in Lozu-Energy Van Hove et d. (op cit.). i i For an overview on W E E D intensity oscillations, see B. A. Joyce, J. H. Neave, J. Zhang, and I? J. Dobson. In: RefkctionNATO (op cit.). 12 I? I. Cohen, l? R. Pukite, J. M. Van Hove, and C. S. Lent.]. Vac. Sci. Tech nol. A4,1251,1986. 13 D. E. Savage and M. G. Lagally. J. Trac. Sei. Echnol B4,943,1986. 14 J. L. Beeby. Reflerrn NATO (op cit.). 6

4.6

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277

ELECTRON EMISSION SPECTROSCOPIES 5.1 5.2 5.3 5.4

5.0

X-Ray Photoelectron Spectroscopy,XPS 282 Ultraviolet Photoelectron Spectroscopy, UPS 300 Auger Electron Spectroscopy,A E S 310 Reflected Electron Energy-Loss Spectroscopy, REELS

324

INTRODUCTION

In this chapter we have collected together those techniques in which one measures the energy distribution of electrons ejected from a material. In all four techniques covered electronic energy level excitations are involved, providing atomic or chemical state identification, or both. All are also true surface techniques, since the energies of the electrons concerned fall in the range where they travel can only very short distances without being inelastically scattered. These techniques are all sensitive to less than monolayer amounts of material and none have probing depths greater than about 50 A without using sputter profiling. The first two techniques discussed, X-Ray Photoelectron Spectroscopy, X P S , (also known as Electron Spectroscopy for Chemical Analysis, ESCA) and Ultraviolet Photoelectron Specrroscopy, UPS, are very closely related. XPS involves soft X rays (usually 1486 eV, from an Al anode) ejecting photoelectrons from the sample. Electrons originating from the core levels identify the elements present from their Binding Energies, BE. Small "chemical shifts" in the BEs provide additional chemical state information. The relative concentrations of the different elements present can be determined from relative peak intensities. XPS identifies all elements except hydrogen and helium from a depth ranging from around 2 monolayers to 25 monolayers. Typical values for X P S peaks in the 500-1400 eV kinetic energy range are 5 to 10 monolayers. 279

The strengths of XPS are its good quantification, its excellent chemical state determination capabilities, its applicability to a wide variety of materials from biological materials to metals, and its generally nondestructive nature. XPS's weaknesses are its lack of good spatial resolution (70 p), only moderate absolute sensitivity (typically 0.1 at. %), and its inability to detect hydrogen. Commercial XPS instruments are usually fully U W compatible and equipped with accessories, including a sputter profile gun. Costs vary from $250,000 to $600,000, or higher if other major techniques are included. UPS differs from X P S only in that it uses lower energy radiation to eject photoelectrons, typically the 2 1.2-eV and 40.8-eV radiation from a He discharge lamp, or up to 200 eV at synchrotron facilities. The usual way to perform UPS is to add a He lamp to an existingX P S system, at about an incremental cost of $30,000. Most activity using UPS is in the detailed study of valence levels for electronic structure information. For materials analysis it is primarily useful as an adjunct to XPS to look at the low-lying core levels that can be accessed by the lower energy UPS radiation sources. There are several advantages in doing this: a greater surfice sensitivity because the electron kinetic energies are lower, better energy resolution because the source has a narrower line width, and the possibility of improved lateral resolution using synchrotron sources. Auger Electron Spectroscopy,AES, is also closely related to XPS. The hole left in a core level after the X P S process, is filled by an electron dropping fiom a less tightly bound level. The energy released can be used to eject another electron, the Auger electron, whose energy depends only on the energy levels involved and not on whatever made the initial core hole. This allows electrons, rather than X rays, to be used to create the initial core hole, unlike XPS. Since all the energy levels involved are either core or valence levels, however, the type of information supplied, like XPS, is elemental identification from peak positions and chemical state information from chemical shifts and line shapes. The depths probed are also similar to XPS. Dedicated AES systems for materials analysis, which are of similar cost to XPS instruments, have electron optics columns producing finely focused, scannable electron beams of up to 30 kV energy and beam spot sizes as small as 200 a great advantage over X P S . A E S could have been discussed in Chapter 3 along with STEM, EMPA, etc. When the incident beam is scanned over the s.mple (Scanning Auger Microprobe, SAM)mapping at high spatial resolution is obtained. For various reasons the area analyzed is always larger than the spot size, the practical limit to S A M being in the 300-1000 A range. Another advantage ofAES over XPS is speed, since higher electron beam currents can be used. There are major disadvantages to using electrons, however. Beam damage is often severe, particularly for organics, where desorption or decomposition often occurs under the beam. Sample charging for insulators is also a problem. Overall, the two techniques are about equally widespread and are the dominant methods for nontrace level analysis at surfaces. AES is the choice for inorganic systems where high spatial resolution is needed (e+ serni280

ELECTRON EMISSION SPECTROSCOPIES

Chapter 5

conductor devices) and XPS should be one’s choice otherwise. Combined systems are quite common. Reflected Electron Energy-Loss Spectroscopy, REELS, is a specialized adjunct to AES, just as UPS is to X P S . A small fraction of the primary incident beam in AES is reflected from the sample surface after suffering discrete energy losses by exciting core or valence electrons in the sample. This fraction comprises the electron energyloss electrons, and the values of the losses provide elemental and chemical state information (the Core Electron Energy-Loss Spectra, CEELS) and valence band information (the Valence Electron Energy-Loss Spectra, VEELS). The process is identical to the transmission EELS discussed in Chapter 3, except that here it is used in reflection, (hence REELS, reflection EELS), and it is most useful at very low beam energy (e.g., 100 eV) where the probing depth is at a very short minimum (as in UPS). Using the rather high-intensity VEELS signals, a spatial resolution of a few microns can be obtained in mapping mode at 100-eV beam energy. This can be improved to 100 nm at 2-keV beam energy, but the probing depth is now the same as for X P S and AES. Like UPS, E E L S suffers in that there is no direct elemental analysis using valence region transitions, and that peaks are often overlapped. The technique is free on any AES instrument and has been used to map metal hydride phases in metals and oxides at grain boundaries at the 100-nm spatial resolution level.

281

5.1

XPS X-Ray PhotoelectronSpectroscopy C. R. B R U N D L E

Contents

Introduction Basic Principles Analysis Capabilities More Complex Effects Surface Sensitivity Instrumentation Applications Comparison with Other Techniques Conclusions

Introduction

The photoelectric process, discovered in the early 1 9 0 0 was ~ ~ developed for analytical use in the 1960s, largely due to the pioneering work of Kai Siegbahn's group.' Important steps were the development of better electron spectrometers, the realization that electron binding energies were sensitive to the chemical state of the atom, and that the technique was surface sensitive. This surface sensitivity, combined with quantitative and chemical state analysis capabilities have made X P S the most broadly applicable general surface analysis technique today. It can detect all elements except hydrogen and helium with a sensitivity variation across the periodic table of only about 30. Samples can be gaseous, liquid, or solid, but the vast majority of electron spectrometers are designed to deal with solids. The depth of the solid material sampled varies from the top 2 atomic layers to 15-20 layers. The area examined can be as large as 1 cm x 1 cm or as small as 70 Prn x 70 Pm (10-pm diam282


Chapter 5

eter spots may be achieved with very specialized equipment). It is applicable to biological, organic, and polymeric materials through metals, ceramics, and semiconductors. Smooth, flat samples are preferable but engineering samples and even powders can be handled. It is a nondestructive technique. Though there are some cases where the X-ray beam damage is significant (especiallyfor organic materials), X P S is the least destructive of all the electron or ion spectroscopy techniques. It has relatively poor spatial resolution, compared to electron-impact and ionimpact techniques. It is also not suitable for trace analysis, the absolute sensitivity being between 0.01-0.3% at., depending on the element. X P S can be a slow technique if the extent of chemical detail to be extracted is large. Analysis times may vary from a few minutes to many hours. There are thousands of commercial spectrometers in use today in materials analysis, chemistry, and physics laboratories. The largest concentrations are in the US and Japan. They are used in universities, the semiconductor and computer industries, and the oil, chemical, metallurgical, and pharmaceutical industries. Instruments combining X P S with one or more additional surface techniques are not uncommon. Such combinations use up relatively little extra space but cost more.

Basic Principles Background

A photon of sufficiently short wavelength (i.e., high energy) can ionize an atom, producing an ejected free electron. The kinetic energy KEof the electron (the photoelectron) depends on the energy of the photon h expressed by the Einstein photoelectric law: KE = h - BE

(1)

where BE is the binding energy of the particular electron to the atom concerned. All of photoelectron spectroscopy is based on Equation (1).Since hv is known, a measurement of KE determines BE. The usefulness of determining BE for materials analysis is obvious when we remember the way in which the electron shells of an atom are built up. The number of electrons in a neutral atom equals the number of protons in the nucleus. The electrons, arranged in orbitals around the nucleus, are bound to the nucleus by electrostatic attraction. Only two electrons, of opposite spin, may occupy each orbital. The energy levels (or eigenvalues E) of each orbital are discrete and are different for the same orbital in different atoms because the electrostaticattraction to the different nuclei (i.e., to a different number of protons) is different. T o a first approximation, the BE of an electron, as determined by the amount of energy required to remove it from the atom, is equal to the E value (this would be exactly true if, when removing an electron, all the other electrons did not 5.1

XPS

283

I

A

I

400

0

I

1200

800

KE(eV)

I

I

I

1200

800

400

I 0

7

BE = hlcKE

b

Figure 1

(a) Schematic representationof the electronic energy levels of a C atom and the photoionizationof a C 1s electron. (b) Schematic of the KEenergy distribution of photoelectrons ejected from an ensemble of C atoms subjected to 1486.6-eV X rays.(c) Auger emission relaxationprocessfor the C 1s hole-state produced in (a).

respond in any way). So, by experimentallydetermininga BE, one is approximately determining an E value, which is specific to the atom concerned, thereby identifying that atom. Photoelectron Process and Spectrum

Consider what happens if, fbr example, an ensemble of carbon atoms is subjected to X rays of 1486.6 eV energy (the usual X-ray source in commercial X P S instruments). A carbon atom has 6 electrons, two each in the Is, 2s,and 2p orbitals, usually written as C IS2 2s’ 2p2. The energy level diagram of Figure l a represents this electronic structure. The photoelectron process for removing an electron from the 284


Chapter 5

1s level, the most strongly bound level, is schematically shown. Alternatively, for any individual C atom, a 2s or a 2p electron might be removed. In an ensemble of C atoms, all three processes will occur, and three groups of photoelectrons with three different KEs will therefore be produced, as shown in Figure 1b where the distribution (the number of ejected photoelectrons versus the kinetic energy)-the photoelectron spectrum-is plotted. Using Equation (11, a BE scale can be substituted for the KE scale, and a direct experimental determination of the electronic energy levels in the carbon atom has been obtained. Notice that the peak intensities in Figure 1b are not identical because the probability for photoejection from each orbital (called the photoionization cross section, o)is different. The probability also varies for a given orbital (e.g., a Is orbital) in different atoms and depends on the Xray energy used. For carbon atoms, using a 1486.6-eV X ray, the cross section for the Is level, oc Is is greater than oc ZS or oc ZP' and therefore the C 1s X P S peak is largest, as in Figure 1b. Thus, the number of peaks in the spectrum corresponds to the number of occupied energy levels in the atoms whose BEs are lower than the X-ray energy hv; the position of the peaks directly measures the BEs of the electrons in the orbitals and identifies the atom concerned; the intensities of the peaks depend on the number of atoms present and on the Q values for the orbital concerned. All these statements depend on the idea that electrons behave independently of each other. This is only an approximation. When the approximation breaks down, additional features can be created in the spectrum, owing to the involvement of some of the passive electrons (those not being photoejected). Analysis Capabilities Elemental Analysis

The electron energy levels of an atom can be divided into two types: core levels, which are tightly bound to the nucleus, and valence levels, which are only weakly bound. For the carbon atom of Figure 1, the C Is level is a core le\7el and the C 2s and 2p levels are valence levels. The valence levels of an atom are the ones that interact with the valence levels of other atoms to form chemical bonds in molecules and compounds. Their character and energy is changed markedly by this process, becoming characteristic of the new species formed. The study of these valence levels is rhe basis of ultraviolet photoelectron spectroscopy (UPS) discussed in another article in this encyclopedia. The core-level electrons of an arom have energies that are nearly independent of the chemical species in which the atom is bound, since they are not involved in the bonding process. Thus, in nickel carbide, the C Is BE is within a few eV of its value for elemental carbon, and the Ni 2p BE is within a few eV of its value for Ni metal. The identification of core-level B f i thus provides unique signatures of the elements. All elements in the periodic table can be identified in this manner, except for H and He, which have no core levels. Approximate 5.1

XPS

285

1400 r

1200 -

f

lo00

-

800

-

600

-

jls

i

IF

io

2 iN w * 400 200

0

10

20

30

40

50

Atomic No. (z)

Figure 2

Approximate BEs of the different electron shells as a function of atomic number Zof the atom concerned, up to the 1486.6-eV limit accessible by AI K a radiation.

BEs of the electrons in all the elements in the period table up to Z= 70 are plotted in Figure 2, as a function of their atomic number 2, up to the usual 1486.6-eV accessibility limit.* Chance overlaps of BEvalues from core levels of different elements can usually be resolved by looking for other core levels of the element in doubt. Quantitative analysis, yielding relative atomic concentrations, requires the measurement of relative peak intensities, combined with a knowledge of 6,plus any experimental artifgcts that affect intensities. Cross section values are known from well-established calc~lations,~ or from experimental measurements of relative peak areas on materials of known composition (standards)? A more practical problem is in correctly determining the experimental peak areas owing to variations in peak widths and line shapes, the presence of subsidiary features (often caused by the breakdown of the independent electron model), and the difficulty of correctly subtracting a large background in the case of solids. There are also instrumental effects to account for because electrons of different KEare not transmitted with equal eKciency through the electron energy analyzer. This is best dealt with by calibrating the instrument using local standards, i.e., measuring relative peak areas for stan286


Chapter 5

c(ls)

Ni (2 p 3 9 rn

300

295

290

865

860

855

850

BE(eV)

a

b

105

100 4-

BE(eV) C

(a) C 1s XPS spectrum from gaseous CF3COCHzCH3., (b)Ni 2pm XPS spectrum from a mixed Ni metal/Ni metal oxide system. (e) Si 2pm XPS spectrum from a mixed Si/SiOz system.

Figure 3

dards of known composition in the same instrument to be used for the samples of unknown composition. Taking all the above into account, the uncertainty in quantification in XPS can vary from a few percent in favorable cases to as high as 30% for others. Practitioners generally know which core levels and which types of materials are the most reliable, and in general, relative differences in composition of closely related samples can be determined with much greater accuracy than absolute compositions. Chemical State Analysis

Though a core level BEis approximately constant for an atom in different chemical environments, it is not exactly constant. Figure 3a shows the C 1s part of the XPS spectrum of the molecule CF3COCHZCH3. Four separated peaks corresponding to the four inequivalent carbon atoms are present.' The chemical shift range ABE covering the four peaks is about 8 eV compared to the BEof -290 eV, or -3%. The carbon atom with the highest positive charge on it, the carbon of the CF3 group, has the highest BE. This trend of high positive charge and high BEis in accordance 5.1

XPS

287

Element

Oxidation state

Chemical shift from zero-valent state

Ni

Ni2+

-2.2 eV

Fe

Fez+

-3.0 eV

Fe3+

-4.1 eV

Ti

Ti4+

-6.0 eV

Si

si4+

-4.0 eV

Al

Al3+

-2.0 eV

cu

cu+

-0.0 eV

cu2+

-1.5 eV

Zn

Zn2+

-0 eV

W

w4'

2 eV

w6'

4 eV

Table 1

Typical chemical shift values for XPS core levels.

with the simplest classical electrostatic representation of the atom as a sphere of radius r with a valence charge q on its surface. The potential inside the sphere q/ r is felt by the 1s electrons. If q increases, the BEof the 1s level increases, and vice versa. This picture is a gross oversimplification because electrons are not so well separated in space, but the general idea that the BE increases with increasing charge on the atom holds in the majority of cases. Table 1 lists the approximate chemical shifts found for the different oxidation states of various metals and semiconductors. The typical range is 1 to several eV, though in some important cases (e.g., Cu and Zn) it is very small. Typical spectra illustrating these chemical shifts for a mixed Ni metal/nickel oxide system and a mixed silicon/silicon dioxide system are shown in Figures 3b and 3c. The spectra of Figure 3 illustrate two hrther points. All the C 1s peaks in Figure 3a are of equal intensity because there are an equal number of each type of C atom present. So, when comparing relative intensities of the same atomic core level to get composition data, we do not need to consider the photoionization cross section. Therefore, Figure 3c immediately reveals that there is four times as much elemental Si present as Si02 in the Si 2p spectrum. The second point is that the chemical shift range is poor compared to the widths of the peaks, especially for the solids in Figures 3b and 3c. Thus, not all chemically inequivalent atoms can be distin288


Chapter 5

guished this way. For example, Cuo (metal) is not distinguishabre from Cu+ in Cu,O, and Zno is not distinguishablefrom Zn2+(e.g., in ZnO). More Complex Effects

In realiry, while the photoelectron is leaving the atom, the other electrons respond to the hole being created. The responses, known as j m l state gects, often lead to additional &tures in the XI’S spectrum, some of which are useful analytically. An effect that always occurs is a lowering of the total energy of the ion due to the relaxation of the remaining electrons towards the hole. This allows the outgoing photoelectron to carry away greater E,i.e., the BEdetermined is always lower than E. This needs to be considered when comparing theoretical E values to experimental BE, i.e., for detailed interpretation of electronic structure effects,but is not generally used analytically. Spin-orbit splitting results from a coupling of the spin of the unpaired electron left behind in the orbital from which its partner has been photoejected with the angular momentum of that orbital, giving two possible different energy final states (spin up or spin down). It occurs for all levels except s levels, which have no orbital angular momentum (being spherical), turning single peaks into doublet p&. The splitting increases with Zl as can be seen from Figure 2 in, for example, the 2 ~ 3 1 2 and 2 p spin-orbit ~ split components of the 2p level. The only analytical usefulness is that the splitting increases the number of X P S peaks per atom in a completely known way, which can help when overlaps occur. Some elements, particularly the transition metals, have unpaired electron spins in their valence levels. The degree of unpairing is strongly affected by the bonding process to other atoms. An unpaired core-electron remaining after the photoemission process will couple to any unpaired spin in the valence level, again leading to more than one final state and peak splitting, called multiplet splitting (weaker than the equivalent spin-orbital splitting). Since the degree of unpaired electron spin in the valence lev& is suongly Acted by chemical bonding, so is the size of the multiplet splitting. For example, rhe Cr (3s) level of the Cr”’ ion of Cr203is split by 4.2 eV, whereas in the more covalent compound CrZS3 the splitting is 3.2 eV, allowing distinction of Cr”’ in the two compounds.’ While a core-electron is being ejected, there is some probabdity that a valence electron will be simultaneouslyexcited to an empty orbital level during the relaxation process, Figure 4b. If this shake-up process occur^, the photoelectron must be ejected with less energy, shifting the X P S peak to apparently higher BE than for a case where shake-up doesn’t occur, as shown in Figure 4c. These “shake-up satellites” in the spectrum are usually weak because the probability of their occurrence is low, but in some cases they can become as strong as the “main” peak Shake-up structure can provide chemical state identification because the valence levels are involved. A typical example is given in Figure 4d. The ion Cu2+(in G O )is distin5.1

XPS

289

e

-

KE

a

b H

u

C

(

2 "MaiA" p 3

/

2

)

"Satellites" CUOfCU'+)

d

CUpOf CU')

945

940

935

930

B.E.(eV)

Figure 4

!Schematic electron energy level diagram: (a) of a core-level photoelectron ejection process (one electron process); (b) core-level photoelectron ejection process with shake-up (two- e l e o n process); (c) schematic XPS spectrum from (a) plus (b); (d) Cu 2133,* XPS spectrum for Cu' in Cu20 and Cu" in CuO. The latter shows strong shake-up features.

guishable from Cu' (in Cu20) by the presence of the very characteristic strong Cu 2p shake-up structure for Cu2+.The chemical shift between Cu2+and Cu+ could also be used for identification, provided accurate BEs are measured. It is sometimes an advantage not to have to rely on accurate BEs, for instance, when comparing data of different laboratories or if there is a problem establishing an accurate value because of sample charging. In such cases the "fingerprinting" pattern identification of a main peak plus its satellites, as in Figure 4d, is particularly useful. M e r the photoemission process is over, the core-hole left behind can eventually be filled by an electron dropping into it from another orbital, as shown in Figure IC for the example of carbon. The energy released, in this example - E ~ ~ may , be 290


Chapter 5

sufficient to eject another electron. The example of a 2p electron being ejected is shown. This is called Auger electron emission and the approximate E of the ejected Auger electron will be

KE(Auger) =

(E1,-&

2P

) -E

2P

The value is characteristic of the atomic energy levels involved and, therefore, also provides a direct element identification (see the article on AES). The E (Auger) is independent of the X-ray energy bv and therefore it is not necessary to use monochromatic X rays to perform Auger spectroscopy. Therefore, the usual way Auger spectroscopy is performed is to use high- energy electron beams to make the coreholes, as discussed in the AES article. We mention the process here, however, because when doing X P S the allowable Auger process peaks are superimposed on the spectrum, and they can be used as an additional means of element analysis. Also, in many cases,chemical shifts of Auger peaks, which have a similar origin to X P S core-level shifts, are larger, allowing chemical state identification in cases where it is not possible directly from the XPS core levels. For example, 2n2+can be whereas it was mentioned distinguishedfrom Zno by a 3-eV shift in Auger peak E, earlier that the two species were not distinguishable using XPS core levels. Surface Sensitivity

Electrons in XPS can travel only short distances through solids before losing energy in collisions with atoms. This inelastic scattering process, shown schematicallyin Figure 5a, is the reason fbr the surfice sensitivity of XPS. Photoelectrons ejected from atoms “very near” the surface escape unscattered and appear in the X P S peaks. Electrons originating from deeper have correspondingly reduced chances of escaping unscattered and mostly end up in the background at lower KE after the X P S peak, as in Figure 5b. Thus, the peaks come mostly from atoms near the surfice, the background mostly from the bulk. If 10 is the flux of electrons originating at depth d the flux emerging without being scattered, Id, exponentially decreases with depth according to -d

where 8 is the angle of electron emission and &sin 8 is the distance travelled through the solid at that angle. The quantity A, is called the inehtic meanfieepatb h g b . The value of A,, which determines quantitatively exactly how surface sensitive the measurement is, depends on the E of the electron and the material through which it travels. Empirical relationships between A, and mare plotted in Figure 6 for elements and for compounds6They are meant as rough guides because values can vary considerably (by a hctor of almost 4), depending on what element 5.1

XPS

291

hu

Vacuum Surface Solid %

.m 4-

c

a,

t

Background Step (Scattered Electrons)

-c 4-

a

Figure 5

b

(a) Schematic of inelastic electron scattering occurring as a photoelectron, initial energy KEo, tries to escape the solid, starting at different depths. KE, c KE3 c KE, c KE, c KE0. (b) KE energy distribution (i.e., electron spectrum) obtained due to the inelastic scattering in (a). Note that the peak, at 4,must come mainly from the surface region, and the background step, consistingof the lower energy scattered electrons, from the bulk.

or compound is involved. Substituting A, values from the curves into Equation (3) tells us that for normal emission (0 = 90") using a 200-eV KE XPS peak, 90% of the signal originates from the top -25 A, for elements. For a 1400-eV peak the depth is -60 A. The numbers are about twice as big for compounds. Thus, the depth probed by XPS varies strongly depending on the XPS peaks used and the material involved. The depth probed can also be made smaller for any given XPS peak and material by detecting at grazing emission angle 8. For smooth surfaces, values down to 10"are practical, for which the depth probed is reduced by a factor of l/sin 10, or -6, compared to 90",from Equation (3).Varying KEor 8 are important practical ways of distinguishing what is in the outermost atomic layers from what is underneath. Instrumentation

An X P S spectrometer schematic is shown in Figure 7. The X-ray source is usually an Al- or Mg-coated anode struck by electrons from a high voltage (1 0-1 5 kv) Alka or Mgka radiation lines produced at energies of 1486.6 eV and 1256.6 eV, with line widths of about 1 eV. The X rays flood a large area (- 1 cm2). The beam's spot size can be improved to about 1OO-pm diameter by focusing the electron beam

292


Chapter 5

1

\ t'

1000 100

-a

-a

10

100

10

I 1

I

10

I

100

KE(eV) Figure 6

Inorganic cpds

000

1000

1

10

100

1000

KE(eV)

Mean free path lengths & as a function of K€, determined for (a) metals and (b)inorganiccompounds.6

onto the anode and passing the X rays through an X-ray monochromator. The latter also improves line widths to between 0.5 and 0.25 eV, leading to higher resolution spectra (thus improving the chemical state identification process) and removing an unwanted X-ray background at lower energies. Practical limits to the shape and size of samples are set by commercial equipment design. Some will take only small samples (e.g., 1 cm x 1 cm) while others can handle whole 8-in computer disks. Flat samples improve signal strength and allow quantitative e variation, but rough samples and powders are also routinely handled. Insulating samples may charge under the X-ray beam, resulting in inaccurate BE determinations or spectra distorted beyond use. The problem can usually be mitigated by use of a low-energy electron flood gun to neutralize the charge, provided this does not damage the sample. The electron lenses slow th'e electrons before entering the analyzer, improving energy resolution. They are also used to define an analyzed area on the sample from which electrons are received into the analyzer and, in one commercial design, to image the sample through the analyzer with 1O-pm tesolution. Older instruments may have slits instead of lenses. The most popular analyzer is the hemispherical sector, which consists of two concentric hemispheres with a voltage applied benveen them. This type of analyzer is naturally suited to varying 8 by rotating the sample, Figure 7. The X P S spectrum is produced by varying the voltages on the lenses and the analyzer so that the trajectories of electrons ejected from the sample at different energies are brought, in turn, to a focus at the analyzer exit slit. A channeltron type electron multiplier behind the exit slit of the analyzer amplifiers individual electrons by 105-106, and each such pulse is fed to external conventional pulse counting electronics and on into a computer. The computer also controls the lens and 5.1

XPS

293

Pul e Counhg Electronics

.

AComputer

Figure 7

UHV Chamber

D Voltage Controls to Lenses, Analyzer

Schematic of a typical electron spectrometer showing all the necessary components. A hemispherical electrostaticelectron energy analyser is depicted.

analyzer voltages. A plot of electron pulses counted against analyzer-lens voltage gives the photoelectron spectrum. More sophisticated detection schemes replace the exit stir-multiplier arrangementwith a multichannel array detector. This is the modern equivalent of a photographic plate, allowing simultaneous detection of a range of KEs, thereby speeding up the detection procedure. Commercial spectrometers are usually bakeable, can reach ultrahigh-vacuum pressures of better than 1O-g Torr, and have fast-entry load-lock systems for inserting samples. The reason for the ultrahigh-vacuum design, which increases cost considerably, is that reactive sudkces, e.g., dean metals, contaminate rapidly in poor yacuum (1 atomic layer in 1 s at 1O4 Torr). If the purpose ofthe spectrometer is to always look at as-inserted samples, which are already contaminated, or to examine rather unreactive surfices (e.g., polymers) vacuum conditions can be relaxed considerably.

294


Chapter 5

Applications

X P S is routinely used in industry and research whenever elemental or chemical state analysis is needed at surfaces and interfaces and the spatial resolution requirements If the analysis is related specifically to the are not demanding (greater than 150 v). top 10 or so atomic layers of air-exposed sample, the sample is simply inserted and data d e n . Examples where this might be appropriate include: examination for and identification of surface contaminants; evaluation of materials processing steps, such as cleaning procedures, plasma etching, thermal oxidation, silicide thin-film formation; evaluation of thin-film coatings or lubricants (thicknessquantity, chemical composition); failure analysis for adhesion between components, air oxidation, corrosion, or other environmental degradation problems, tribological (wear) activity; effectiveness of surface treatments of polymers and plastics; surface composition differences for alloys; examination of catalyst surfaces before and after use, after “activation”procedures, and unexplained hilures. Figure 3c was used to illustrate that Si’” could be distinguished from Sio by the Si 2p chemical shift. The spectrum is actually appropriate for an oxidized Si wafer having an 10-A Si02 overlayer. That the Si02 is an overlayer can easily be proved by decreasing 8 to increase the surfgcesensitivity; the Siosignal will decrease relative to rhe Siw signal. The 10-Athickness can be determined from the Si”/Si0 ratio and Equation (3), using the appropriate 4 value. That the overlayer is Si02 and not some other Si’” compound is easily verified by observing the correct position (BE) and intensity of the 0 1s peak plus the absence of other element peaks. If the sample has been exposed to moisture, including laboratory air, the outermost atomic layer will actually be hydroxide, not oxide. This is easily recognized since there is a chemical shift between OH and 0 in the 0 1s peak position. Figure 8 shows a typical example where surface modification to a polymer can be f ~ l l o w e d .High-density ~ polyethylene (CHlCH,), was surface-fluorinated in a dilute fluorine-nitrogen mixture. Spectrum A was obtained after only 0.5 s treatment. A F 1s signal corresponding to about a monolayer has appeared, and CF formation is obvious from the chemically shifted shoulder on the C 1s peak at the standard CF position. After 30 s reaction, the F 1s / C 1s ratio indicates (spectrum B) that the reaction has proceeded to about 30 A depth, and that CF2 formation has occurred, judging by the appearance of the C 1s peak at 291 eV. Angular studies and more detailed line shape and relative intensity analysis, compared to standards, showed that for the 0.5-s case, the top monolayer is mainly polyvinyl fluoride (CFHCHZ),, whereas after 30 s polytrifluoroethylene (CFZCFH), dominates in the top two layers. While this is a rather aggressive example of surface treatment of polymers, similar types of modifications frequently are studied using X P S . An equivalent example in the semiconductor area would be the etching processes of Si/SiO2 in CF4/02 mixtures, where varying the CFs/02 ratio changes the relative etching rates of Si and Si02, and also produces different and varying amounts of residues at the wafer’s surface.

-

5.1

XPS

295

CH

1

A 691

289

687

285

BE(eV)

Figure 8

XPS spectrum in the C I s and F 1s regions of polyethylene(CH2)., treated with II dilute Fz/N2 gaseous mixturefor (a) 0.5 set, and (b) 30 set?

In many applications the problem or propconcerned is not related just to the top 10 or so atomic layers. Information from deeper regions is required for a number of reasons: A thick contaminant layer, caused by air exposure, may have of interest; the material may be a layered structure in which covered up the&s the buried interfaces are important; the composition modulation with depth may be important, etc. In such cases, the 2-1 5 atomic layer depth resolution attainable in X P S by varying 8 is insufficient, and some physical means of stripping the su& while taking data, or prior to taking data, is required. This problem is common to a l l very surfice sensitive spectroscopies. The most widely used method is argon ion sputtering, done inside the spectrometerwhile taking data. It can be used to depths of pm, but is most effective and generally used over mudl shorter distances (hundreds and thousands of Hi> because it can be a slow process and because sputtering introduces artifacts that get worse as the sputtered depth increases.8 These indude interf$cialmixing caused by the movement of atoms under the Ar' beam, elemental composition alteration caused by preferential sputtering of one element versus another, and chemical changes caused by bonds being broken by the sputtering ProCeSS. If the interface or depth of interest is beyond the capabilityofsputtering, one can try polishing down, sectioning, or chemical etching the sample before insertion. 296


Chapter 5

The effectivenessof this approach varies enormously, depending on the material, as does the extent of the damaged region left at the surface after this preparation treatment. In some cases, the problem or property of interest can be addressed only by performing experiments inside the spectrometer. For instance, metallic or alloy embrittlement can be studied by fracturing samples in ultrahigh vacuum so that the fractured sample surface, which may reveal why the fracture occurred in that region, can be examined without air exposure. Another example is the simulation of processing steps where exposure to air does not occur, such as many vacuum deposition steps in the semiconductor and thin-film industries. Studying the progressive effects of oxidation on metals or alloys inside the spectrometer is a fiirly well-established procedure and even electrochemical cells are now coupled to X P S systems to examine electrode surfaces without air exposure. Sometimes materials being processed can be capped by deposition of inert material in the processing equipment (e.g., Ag, Au, or in GaAs work, arsenic oxide), which is then removed again by sputtering or heating after transfer to the X P S spectrometer. Finally, attempts are sometimes made to use “vacuum transfer suitcases” to avoid air exposure during transfer.

Comparison with other Techniques

X P S , AES, and SIMS are the three dominant surface analysis techniques. X P S and

A E S are quite similar in depth probed, elemental analysis capabilities, and absolute sensitivity. The main X P S advantages are its more developed chemical state analysis capability, somewhat more accurate elemental analysis, and far fewer problems with induced sample damage and charging effects for insulators. A E S has the advantage of much higher spatial resolutions (hundreds of A compared to tens of pm), and speed. Neither is good at trace analysis, which is one of the strengths of SIMS (and related techniques). SIMS also detects H, which neither AES nor X P S do, and probes even less deeply at the surface, but is an intrinsically destructive technique. Spatial resolution is intermediate between AES and X P S . ISS is the fourth spectroscopy generally considered in the “true surface analysis” category. It is much less used, partly owing to lack of commercial instrumentation, but mainly because it is limited to elemental analysis with rather poor spectral distinction between some elements. It is, however, the most surface sensitive elemental analysis technique, seeing only the top atomic layer. With the exception of EELS and H E E L S , all other spectroscopies used for surface analysis are much less surface sensitive than the above four. H E E L S is a vibrational technique supplying chemical functional group information, not elemental analysis, and EELS is a rarely used and specialized technique, which, however, can detect hydrogen.

5.1

XPS

297

Conclusions

X P S has developed into the most generally used of the truly surface sensitive techniques, being applied now routinely for elemental and chemical state analysis over a range of materials in a wide variety of technological and chemical industries. Its main current limitations are the lack of high spatial resolutions and relatively poor absolute sensitivity (i.e., it is not a trace element analysis technique). Recently introduced advances in commercial equipment have improved speed and sensitivity by using rotating anode X-ray sources (more photons) and parallel detection schemes. Spot sizes have been reduced from about 150 pm, where they have languished for several years, to 75 pm. Spot sizes of 10 pm have been achieved, and recently anounced commercial instruments offer these capabilities. When used in conjunction with focused synchrotron radiation in various “photoelectron microscope” modes higher resolution is obtainable. Routinely available 1 pm X P S resolution in laboratory-based equipment would be a major breakthrough, and should be expected within the next three years. Special, fully automated one-task XPS instruments are beginning to appear and will find their way into both quality control laboratories and process control on production lines before long. More detailed discussions of XPS can be found in references 4-12, which encompass some of the major reference texts in this area. Related Articles in the Encydopedia

UPS, AES, SIMS, and ISS References I

2

3

K. Siegbahn et al. ESG4: Atomic, Molecular, andSolid State Structure Studied by Means ofElectron Spectroscopy. Nova Acta Regime SOC.Sd.,Upsaliensis, 1967, Series IV, Volume 20; and K. Siegbahn et al. E S U Applied to Free Molecules. North Holland, Amsterdam, 1969. These two volumes, which cover the pioneering work of K.Siegbahn and coworkers in developing and applying X P S , are primarily concerned with chemical structure identification of molecular materials and do not specifically address surface analysis. Charts such as this, but in more detail, are provided by all the X P S instrument manufacturers. They are based on extensive collections of data, much of which comes from Reference 1. J. H. Scofield. J Electron Spect. 8,129, 1976.This is the standard quoted reference for photoionization cross sections at 1487eV. It is actually one of the most heavily cited references in physical science. The calculations are published in tabular form for all electron level of all elements.

298


Chapter 5

See, for example, S. Evans et a1.J E l e m Speck 14,341, 1978. Relative experimental ratios of cross sections for the most intense peaks of most elements are given. 5 J. C. Carver, G. K. Schweitzer, andT. A. Car1son.J Chm. Phys. 57,973, 1972. This paper deals with multiplet splitting effects, and their use in distinguishing different element states, in transition metal complexes. 6 M. E Seah and W. A. Dench. Su$ Inte6a.e Anal. 1, 1,1979. Of the many compilations of measured mean free path length versus m, this is the most thorough, readable, and useful. 7 D. T. Clark, W. J. Feast, W K. R Musgrave, and I. Ritchie. J Polym. Sri. Polym. Chem. 13,857, 1975. One of many papers from Clark's group of this era which deal with all aspects of X P S of polymers. 8 See the article on surface roughness in Chapter 12. 9 The book series Electron Spectroscopy: Theory, Techniques, andApplications, edited by C. R. Brundle and A. D. Baker, published by Academic Press has a number of chapters in its 5 volumes which are usefd for those wanting to learn about the analytical use of XPS: In Volume 1, An Introduction to Ekctron Spectroscopy (Baker and Brundle); in Volume 2, Basic Concepts of XPS (Fadley); in Volume 3, AnalyticalApplicationr ofxPS (Briggs); and in Volume 4, XPSfor the Investigation ofPolymeric Materialj (Dilks). i o T. A. Carlson, Photoelectron andAuger Spectroscopj Plenum, 1975. A complete and largely readable treatment of both subjects. 11 PracticaISufaceAmlysis, edited by D. Briggs and M. E Seah, published by J. Wiley; Handbook ofXPSand UPS, edited by D. Briggs. Both contain extensive discussion on use of XPS for surfice and material analysis. 12 Handbook ofxPS, C. D. Wagner, published by PHI (Perkin Elmer). This is a book of X P S data, invaluable as a standard reference source.

5.1

XPS

299

5.2

UPS Ultraviolet PhotoelectronSpectroscopy C . R. BRUNDLE

Contents

Introduction Basic Principles Analysis Capabilities Conclusions

Introduction

The photoelectric process, which was discovered in the early 1900s was developed as a means of studying the electronic structure of molecules in the gas phase in the early 1960s, largely owing to the pioneering work of D. W. Turner's group.' A major step was the introduction of the He resonance discharge lamp as a laboratory photon source, which provides monochromatic 2 1.2-eV light. In conjunction with the introduction of high resolution electron energy analyzers, this enables the binding energies (BE) of all the electron energy levels below 21.2 eV to be accurately determined with sufficient spectral resolution to resolve even vibrational excitations. Coupled with theoretical calculations, these measurements provide information on the bonding characteristics of the valence-level electrons that hold molecules together. The area has become known as ultraviolet photoelectron spectroscopy (UPS) because the photon energies used (21.2 eV and lower) are in the vacuum ultraviolet (UV) part of the light spectrum. It is also known as molecular photoelectron spectroscopy, because of its ability to provide molecular bonding information. In parallel with these developments for studying molecules, the same technique was being developed independently to study solids: particularly metals and semi300


Chapter 5

conductors.’ This branch of the technique is usually known as UV photoemission. Here the electronic structure of the solid (the band structure for m e d s and semiconductors) was the interest. Since the technique is sensitive to only the top few atomic layers, the electronicstructure of the surfice, which in general can be different from that ofthe bdk, is actually obtained. The two branches of UPS,gas-phase and solid-surface studies, come together when adsorption and reaction of molecules at surfices is studied?. Though commercial UPS instruments were sold in the 1970s,for gas-phase work, none are sold today. Since the only additional item required to perform UPS on an X P S instrument is a He source, this is usually how UPS is performed in the laboratory. An alternative, more specialized approach, is to couple an electron spectrometer to the beam-line monochromator of a synchrotron ficility. This provides a tunable source oflight, usually between around 10 eV and 200 eV, though many beam lines can obtain much higher energies. This approach can provide a number of advantages, including variable surface sensitivity and access to core levels up to the photon energy used, at much higher resolution than obtainable by laboratory X P S instruments. Even using a laboratory UPS source, such as a He resonance lamp, some low-Iying core levels are accessible. When using either synchrotron or laboratory sources to access core levels, all the materials surface analysis capabilities of XPS described in the preceding article become available.

Basic Principles The photoionization process and the way it is used to measure BEs of electrons to afoms is described in the article on X P S and will not be repeated here. Instead, we will concentrate on the differences between the characteristics of core-level BEs, described in the X P S article, and those of valence-level BEs. In Figure l a the electron energy-level diagram for a CO molecule is shown, schematically illustrating how the atomic levels of the C and 0 atom interact to fbrm the CO molecule. The important point ro note is that whereas the BEs of the C 1s and 0 1s core levels remain characteristic of the atoms when the CO molecule is formed (the basis of the use of X P S as an elemental analysis tool), the C 2p and 0 2p valence levels are no longer characteristic of the individual atoms, but have combined to form a new set of molecular orbitas entirely characreristic of the CO molecule. Therefore, the UPS valence-band spectrum of the CO molecule, Figure lb, is also entirely characteristic of the molecule, the individual presence of a C arom and an 0 atom no longer being recognizable. For a solid, such as metallic Ni, the valence-level electrons are smeared out into a band, as can be seen in the UPS spectrum ofNi (Figure 2a). For molecules adsorbed on surfaces there is also a smearing out of structure. For example, Figure 2b shows a monolayer of CO adsorbed on an Ni surface. 5.2

UPS

30;

o c o c

UPS Spectrum (He11

t

--

38t

2u

1s

1s

1U

a

Figure 1

t

295 545 -

b

(a) Electron energy diagram for the CO molecule, illustrating how the molecular orbitals are constructedfrom the atomic levels. (b) He I UPS spectrum of CO.’

Analytical Capabilities

As stated earlier, the major use of UPS is not for materials analysis purposes but for electronic structure studies. There are analysis capabilities, however. We will consider these in two parts: those involving the electron valence energy levels and those involving low-lying core levels accessible to UPS photon energies (including synchrotron sources). Then we will answer the question “why use UPS if X P S is available?” Valence Levels

The spectrum of Figure 1b is a fingerprint of the presence of a CO molecule, since it is different in detail from that of any other molecule. UPS can therefore be used to identify molecules, either in the gas phase or present at surfaces, provided a data bank of molecular spectra is available, and provided that the spectral features are sufficiently well resolved to distinguish between molecules. By now the gas phase spectra of most molecules have been recorded and can be found in the literature. Since one is using a pattern of peaks spread over only a few eV for identification purposes, mixtures of molecules present will produce overlapping patterns. How well mixtures can be analyzed depends, obviously, on how well overlapping peaks can be resolved. For molecules with well-resolved fine structure (vibrational) in the spectra (see Figure lb), this can be done much more successfully than for the broad, ‘3

302


Chapter 5

1

d-band N

i

w

I

NI 3d-level

J I

EF

Figure 2

I

I

I

4

8

12

B.E. (eV)

(a1 He II UPS spectrum of a Ni surface! (bl He II UPS spectrum of a CO monolayer adsorbed on a Ni surface! Note the broadening and relative bindingenergy changes of the CO levels compared to the gas phase spectrum. Gasphase binding energies were measured with respect to the vacuum level; solid state binding energies relative to the Fermi level 6.

unresolved bands found for solid surhces (see Figure 2b). For solids that have electronic structure characteristics in between those of molecules and metals, such as polymers, ionic compounds, or molecules adsorbed on surfaces (Figure 2b), enough of the individual molecular-like structure of the spectra often remains for the valence levels to be used for fingerprinting purposes. Reactions between molecules and surfaces often can be fingerprinted also. For example, in Figure 3 the UPS differences between molecular H,O on a metal, and its only possible dissociation fragments, OH and atomic 0,are schematically illustrated. The examples of valencelevel spectra given so far, for solid surfaces, i.e., those in Figures 2a, 2b, and 3, are all angk-integratedspectra;that is, electrons emitted over a wide solid angle of emission are collected and displayed. In fact, the energy distribution of photoemitted electrons from solids varies somewhat depending on the direction of emission and if data is taken in an angular-resolved mode, that is, for specific directions for the photon beam and the photoemitted electrons, detailed information about the three-dimensional (3D) band structure of the solid, or the two-dimensional (2D) band struczure of an adsorbate overlayer may be obtained, together with information on the geometric orientation of such adsorbate mole5.2

UPS

303

'Typical' metal spectrum plus adsorbed monolayer of MO

Er

4

8

12

MO dissociated to OH adsorbed species

\.I

EF

I

4

I

8

I

12

Com lete dissociation to aPomic oxygen sprcles

E,

4

8

12

B.E. (eV)

Figure3

Schematic spectra of H20, OH, and atomic 0 adsorbed on a metal surface illustrate how moleculescan be distinguishedfrom their reactor products by fingerprinting.

d e s . T o properly exploit the technique requires also variation of the photon energy, h (therefore requiring synchrotron radiation) and the polarizatian of the radiation (s and p, naturally available from the synchrotron source). Basically, recording the UPS spectrum while varying all these parameters (angle, photon energy, and polarization) picks out specific parts of the density of states. A fuller description of this type of work' is beyond the scope of this article and is not particularly relevant to materials analysis, except for the fact that molecular orientation at surfices can be determined. This property is, however, restricted to situations with long-range order, i.e., 2D arrays of molecules on single-crystal surfaces. Low-Lying Accessible Core Levels

Table I lists core levels and their BEs for elements commonly used in technology, which are sufficientlysharp and intense, and which are accessible to laboratory He I or He I1 sources (21.2-eV or 40.8-eV photon energy) or to synchrotron sources (up to 200 eV or higher). The analytical approaches are the same as described in the X P S article. For example, in that article examples were given of Si 2p spectra obtained using a laboratory Al Ka X-ray source at 1486-eV photon energy. The 304


Chapter 5

Core level

Approximate binding energy (eV)

Usable r radiation

AI

2P

72

S

Si

2P

100

S

S

2P

164

S

Zn

3d

9

He 1,He 11,s

Ga

3d

18

He 1,He 11,s

Ge

3d

29

He 11, S

As

3d

41

S

Ta

4f

25

He 11, S

W

4f

34

S

Ir

4f

60

S

Pt

4f

70

S

Au

4f

84

S

Hg

4f

99

S

5d

7

He I, He 11,s

4f

138

S

Element

Pb Table 1

Narrow, intense core levels of some elements commonly used in technological materials that are accessible to He I /He II radiation, or synchrotron radiation below 200 eV.

Si 2p line, at about 100 eV BE, is also easily accessible at most synchrotron sources but cannot, of course, be observed using He I and He I1 radiation. O n the other hand, the Zn 3d and Hg 4f lines can be observed quite readily by He I radiation (see Table 1) and the elements identified in this way. Quantitative analysis using relative peak intensities is performed exactly as in X P S , but the photoionization cross sections CY are very different at UPS photon energies, compared to AI K a energies, and tabulated or calculated values are not so readily available. Quantitarion, therefore, usually has to be done using local standards.

5.2

UPS

305

sio

Si" (Si& wedayor)

(eiernnntai si)

A"[ S P (SI03

S a rnchmtmn 1 M ev mdlation

I 105

,

I

I

I

I

,

I

100

B.E. (eV)

Figure4

Schematic comparison of the Si 2p spectra of an Si/Si02 interface taken using AI K radiation at 1486 eV and synchrotron radiation at 40 eV photon energy. Note the greater surface sensitivity and higher resolution in the synchrotroncase.

W h y Use UPS for Analysis7

Since all the valence levels and core levels that are accessible to UPS photon sources are also accessible to XPS, what are the reasons for ever wanting to use laboratory He sources or synchrotron radiation?There are at least four significant differences that can be important analytically in special circumstances. First, the surface sensitivity is usually greater in UPS because for a given energy level being examined, the lower photon energy sources in UPS yield ejected photoelectrons having lower kinetic energies. For example, the Si 2p signal of Figure 3 in the XPS article consists of electrons having a kinetic energy 1486-100 eV = 1386 eV. If the Si 2p spectrum were recorded using 140 eV synchrotron photons, the kinetic energy would be 140-100 eV = 40 eV. Looking at the inelastic mean-free path length diagram of Figure 6 in the X P S article, one can see that 40-eV photoelectrons have about onethird the inelastic scattering length of 1400-eV electrons. Therefore the synchrotron recorded signal would be roughly three times as surface sensitive, as illustrated in Figure 4 where the XPS Si02 / Si spectrum is schematically compared for 1486 eV and 140 eV photon sources. The Si02 part of the Si 2p signal is much stronger in the synchrotron spectrum and therefore much thinner layers will be more easily detectable. Secondly, spectral resolution can be significantly higher for UPS or synchrotron data, compared to XPS. This is simply a consequence of UPS (synchrotron)sources 306


Chapter 5

having narrower line widths than laboratory X-ray sources. Thus, whereas the XPS recorded Si 2p signal of Figure 4 has a width of about 1 eV, the individual 2 ~ 3 , ~ and 2p% components of the synchrotron recorded signal are only about 0.25 eV wide. Whether this resolution improvement can be achieved in any individual case depends on the natural line width of the particular core level concerned. Si 2p, W 4f, A 2 p , Pt 4f, and Au 4f are all examples of narrow core lines, where a large resolution improvement would occur using synchrotron sources, allowing small chemi d shifts corresponding to chemically distinct species to be more easily seen. For valence levels, higher resolution is also an obvious advantage since, as described earlier, one is usually looking at several lines or bands, which may overlap significantly. Two additional practical points about resolution also should be noted. The spectral resolution of the gratings used to monochromatize synchrotron radiation gets worse as the photon energy gets higher, so the resolution advantage of synchrotron radiation decreases as one goes to high BE core levels. Second, monochromators can be used with laboratory X-ray sources, improving XPS resolution significantly, but not to the degree achievable in UPS or synchrotron work. The third significantdifference between UPS and XPS, from an analytical capability point of view, concerns signal strength. T o zeroth order, CT values are a maximum for photon energies just above photoionization threshold, and then decrease strongly as the photon energy is increased, so valence levels in particular have much greater B values using UPS or synchrotron sources, compared to XPS. When coupled with the high photon fluxes available from such sources, this results in greater absolute sensitivity for UPS or synchrotron spectra. Taking these differences together, one can see that all three work in favor of UPS or synchrotron compared to X P S when trying to observe very thin layers of chemically distinct material at the surface of a bulk material: improved surface sensitivity; improved resolution allowing small surface chemically shifted components in a spectrum to be distinguished from the underlying bulk signal; and improved absolute sensitivity. As a practical matter, one has to ask whether the core levels one wants to use are even accessible to UPS or synchrotron and whether the need to go to a national facility on a very access-limited basis can compare to day-in, day-out laboratory operations. For UPS using He I and He I1 radiation sources the addition of these to existing XPS system is not excessivelycostly and is then always there to provide additional capability useful for specific materials and problems. The final difference between UPS or synchrotron capabilities and XPS, from an analytical point of view, is in lateral resolution. Modern laboratory X P S small-spot instruments can look at areas down to 30-150 p,depending on the particular instrument, with one very specialized instrument offering imaging capabilities at 10-pn resolution, but with degraded spectroscopy capabilities.*For UPS and synchrotron radiation, much higher spatial resolution can be achieved, partly because the lower kinetic energy of rhe photoelectron lends itself better to imaging schemes and partly because of efforts to focus synchrotron radiation to small spot sizes. The 5.2

UPS

307

potential for a true photoelectron microscope with sub 1000-A resolution therefore exists, but it has not been realized in any practical sense yet. Conclusions

UPS, if defined as the use of He I, He 11, or other laboratory low-energy radiation sources (e50ev), has rather limited materials surface analysis capabilities. Valence and core electron energy levels below the energy of the radiation source used can be accessed and the main materials analysis role is in providing higher resolution and high surface sensitivity data as a supplement to X P S data, usually for the purpose of learning more about the chemical bonding state at a surface. Angle-resolved UPS can supply molecular orientation geometric information for ordered structures on single crystal surfaces, but its main use is to provide detailed band structure infixmation. Synchrotron radiation can be used to provide the same information, but also has the great advantage of a wider, tunable, photon energy range. This allows one to access some core levels at higher resolution and surface sensitivity than can be done by XPS. The variable energy source also allows one to vary the surface sensitivity by varying the kinetic energy of the ejected photoelectrons, thereby creating a depth profding capability. Most synchrotron photoemission work to date has involved fundamental studies of solid state physics and chemistry, rather than materials analysis, albeit on such technologically important materials as Si, GaAs, and CdTeHg. Some quite applied work has been done related to the processing of these materials, such as studying the effects of cleaning procedures on residual surface contaminants, and studying reactive ion-etching mechanisms.’ The major drawback of synchrotron radiation is that it is largely unavailable to the analytical community and is an unreliable photon source for those who do have access. As the number of synchrotron facilities increase and as they become more the domain of people wanting to use them as dedicated light sources, rather than in high-energy physics collision experiments, the situation for materials analysis will improve and the advantages over laboratory-based X P S will be more exploitable. Synchrotron radiation will never replace laboratory-based XPS, however, and it should be regarded as complementary, with advantages to be exploited when really needed. High spatial resolution photoelectron microscopy is likely to become one such area. Related Articles in the Encyclopedia

X P S and SEXAFS References 1

D. W. Turner, C. Baker, A. D. Baker, and C. R Brundle. Molecular Pbotoelectron Spectroscop~Wiley, London, 1970.This volume presents a brief

308


Chapter 5

introduction to the principles of UPS and a large collection of spectra on small molecules, together with their interpretation in terms of the electronic structure and bonding of the molecules.

z W. E. Spicer. In Suwey ofPhenomena in Ionized Gases. International Atomic Energy Agency, Vienna, 1968, p. 271. A review of the early photoemission work on solids by the pioneering group in this area. 3

4

D. Menzel.J Vac. Sci. Tech. 12,313, 1975. A review of the applications of UPS to the adsorption of molecules at metal surfaces. C. R. Brundle. In Mokcuhr Spectroscopy(A.R West, Ed.) Heyden, London, 1976.This review discusses both the use ofXPS and UPS in studying adsorption and reactions at surfaces.

5

K. Kimura, S. Katsumata, Y. Achita, Y. Yamazaki, and S. Iwata. Handbook of He I Photoelectron Spectra of Funhmental Organic Molecuks. Halsted Press, New York, 1981. This volume collects together spectra and interpretation for 200 organic molecules.

6

Photoemission in Solid. (L. Ley and M. Cardona, Eds.) Springer-Verlag, New York, 1978 and 1979, Vols. 1 and 2.

7

N. V. Smith and E J. Himpsel. In Handbook on Synchrotron Radiation. (E. E. Koch, Ed.) North Holland, New York, 1983, Vol. 1b. l? Coxon, J. Krizek, M. Humpherson, and I. R. M. Wardell. J Ekc. Spec.

8

821,1990. 9

5.2

J. A. Yarmoff and E R McFeely, Surface Science 184,389, 1987.

UPS

309

5.3

AES Auger Electron Spectroscopy Y.E. STRAUSSER

Contents

Introduction Basic Principles of Auger Information in Auger Spectra Methods for Surface and Thin-Film Characterization Artifacts That Require Caution Conclusions

Introduction

Auger electron spectroscopy (AES) is a technique used to identify the elemental composition, and in many cases, the chemical bonding of the atoms in the s u h c e region of solid samples. It can be combined with ion-beam sputtering to remove material from the surface and to continue to monitor the composition and chemistry of the remaining surface as this surface moves into the sample. It uses an electron beam as a probe of the sample surface and its output is the energy distribution of the secondary electrons released by the probe beam from the sample, although only the Auger electron component of the secondaries is used in the analysis. Auger electron spectroscopy is the most frequently used surface, thin-film, or interfie compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity-a sampling depth that varies 310


Chapter 5

between 5 and 100 A depending upon the energy of the Auger electrons measured and the signal-to-noise ratio in the spectrum. It has good lateral spatial resolution, which can be as low as 300 A, depending on the electron gun used and the sample material. It has very good depth resolution, as low as 20 A depending on the characteristics of the ion beam used for sputtering. It has a good absolute detectability, as low as 100 ppm for most elements under good conditions. It can produce a three-dimensional map of the composition and chemistry of a volume of a sample that is tens of pm thick and hundreds of pm on a side. On the other hand, AES cannot detect H or He. It does not do nondestructive depth profding. It uses an electron beam as a probe, which can be destructive to some samples. It requires the sample to be put into and to be compatible with high vacuum. Some nonconducting samples charge under electron beam probing and cannot be analyzed. The sputtering process can alter the surface composition and thereby give misleading results. It does turn out to be the technique of choice, in its area, much of the time. The purpose of this article is to make clear what it can and cannot do and how to get the most information from it. The Auger process, which produces an energetic electron in a radiationless atomic transition, was first described by Pierre Auger in 1923.’ The detection of Auger electrons in the secondary electron energy spectra produced by electron bombardment of solid samples was reported by J. J. Lander in 1953.2Its use in an analytical technique to characterize solid surfices was made practical by Larry Harris’ analog detection circuitry in 1967.3From that time the technology developed very rapidly, and the technique gained momentum through the 1970s and 1980s. As the technique developed so did the instrumentation. The hardware development has taken advantage of improvements in ultrahigh vacuum technology and computerization. Systems are available having 300-A diameter field emission electron beams; user-friendly, rapidly attained ultrahigh vacuum; and complete computer control of the system. At the other end of the price range are components that can be “plugged in” to various deposition and processing systems to provide in-situ surface characterization. AES, X-Ray Photoelectron Spectroscopy (XI’S), Secondary Ion Mass Spectroscopy (SIMS!, and Rutherford Backscattering Spectroscopy (RBS) have become the standard set of surface, thin-film, and interface analysis tools. Each has its own strengths, and mostly they are complementary. X P S uses X rays as a probe, which are usually less damaging to the surface than the electron beam of Auger but which can’t be focused to give high lateral spatial resolution. X P S is also more ofien selected to determine chemical information. SIMS can detect H and He and has a much higher absolute sensitivity in many cases, but seldom gives any chemical information and, by its nature, has to remove material to do its analysis. RBS readily produces good quantitative results and does nondestructive depth profiling, but it lacks the absolute sensitivity of Auger to many of the important elements and its depth resolution is not as good as Auger can produce, in many cases. 5.3

AES

31 1

Elastic Peak

In I

1

True /Secondaries

I

Cond. Band

Loss Tail

I 0

K

1844-

Energy Energy Levels

Denslty Auger of Transitlons States

a Figure 1

Secondary Electron Background

-

I

E

b

(a) Energy level diagram of solid Si, including the density of states of the valence and conduction bands, a schematic representation of the Si K b , 3 b 3 Auger transitions, and a subsequent L W Auger transition. (b) The complete Secondary electron energy distribution produced by the interaction of a primary electron beam of energy €with a solid surface. The true secondarypeak, the elastic peak, and some Auger peaks are shown. Also shown are the sectail contributions t o the background from ondary background and the IOU each of the Auger peaks.

Basic Principlesof Auger

The basic Auger process involves the production of an atomic inner shell vacancy, usually by electron bombardment, and the decay of the atom from this excited state by an electronic rearrangement and emission of an energetic electron rather than by emission of elecrromagnetic radiation. For example, as illustrated in Figure la, if a Si surface is bombarded by 5-keV electrons, some of the Si atoms will lose electrons from their K shell, whose binding energy is 1.8 keV. The K shell vacancy will typically be filled by the decay of an electron from one of the L subshells, let's say the L2,3 shell, which has a binding energy of 104 eV. This leaves an energy excess of 1.7 keV. This is sometimes relieved by the emission of a 1.7-keVX ray, which is the basis for the EDS and WDS techniques used in'the SEM. Most of the time, however, it is relieved by the ejection of another L2,3 shell electron that overcomes its 0.1-keV binding energy and carries off the remaining 1.6 keV of energy. This characteristic energy is the basis for the identification of this electron as having come from a Si atom in the sample. This electron is called a Si IU2,3L2,3Auger electron and the process is called a JSLL Auger transition. This process leaves the atom with 2 vacancies in the L2,3 shell that may further decay by Auger processes involving +

312


Chapter 5

electrons from the Si M shell, which is also the valence band, and thus these Auger transitions are called L W transitions. The two valence-band electrons involved in an L W transition may come from any two energy states in the band, although they will most probably come from near peaks in the valence-band density of states, and thus the shape of the L W “peak” is derived from a self convolution of the valenceband density of states, and the width of the L W peak is twice the width of the valence band. The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of inner shell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from these inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. In fact, Auger electrons are generated in transitions back to the ground state of atoms with inner shell vacancies, no matter what process produced the inner shell vacancy. Auger peaks are therefore observed in electron energy spectra generated by electron excitation,X-ray excitation, and ion excitation, as well as in certain nuclear reactions. The technique usually referred to as Auger electron spectroscopy uses excitation by an electron beam. The spectra produced by X-ray excitation in X P S routinely also include Auger peaks mixed in with the photoelectron peaks. Ion beam-induced Auger peaks occur, at times, during the depth profding mode of analysis in AES. Production of Inner Shell Vacancies

The probability (cross section) that a high-energy incident electron will produce a particular inner shell vacancy in a certain element is a function of the ratio of the primary electron energy to the binding energy of the electrons in that shell. In general the cross section rises steeply from 0 at a ratio of 1 to a maximum at a ratio in the range from 3 to 6 and then decreases gradually as the ratio increases further. As an example, the Si K shell binding energy is 1844 eV. To get the maximum yield of Si K shell vacancies, and therefore Si KLL Auger electrons, a primary electron-beam energy of 5.5-1 1.0 keV should be used. On the other hand if better surfice sensitivity is needed (see below) the low-energy Si L W rransirion is preferred. The Si L shell binding energies are 154 and 104 eV, so the primary beam energy would be optimized at 0.3-0.9 keV for these transitions. Auger Electron Emission

Once an inner shell vacancy is created in an atom the atom may then return toward its ground state via emission of a characteristic X ray or through a radiationless Auger transition. The probability of X-ray emission is called the fluorescenceyield. 5.3

AES

313

0

15

30

45

60

75

90

Atomic Number 2 Figure 2

Percentage of inner shell vacancies resulting in Auger electron emission for holes in the K, L, and M shells.

The Auger yield is 1 minus the fluorescence yield, since these are the only two options. Figure 2 shows the Auger yield as a hnction of atomic number for initial vacancies in the K, L, and M shells. It is dear that Auger emission is the preferred decay mechanism for K shell vacancies in the low atomic number elements, and for L and M shell initial vacancies for all elements. By properly selecting the Auger transition to monitor, all elements (except H and He) can be detected using Auger transitions that have a 90% or higher Auger yield per initial vacancy. Electron Transport to the Surface

As the various electrons, induding Auger electrons, resulting from primary electron bombardment diffuse through the sample and to the surfice many scatteringevents occur. The inelastic collisions have the effect of smoothing the energy distribution of these electrons and result in a power law energy distribution4at energies between the elastic peak and the “true secondary” peak, which occur at the high-energy and low-energy end of the distribution, respectively. This produces a background, as shown in Figure 1byon which the Auger peaks are superimposed, that can be modeled and removed (see below). Inelastic collisions also have the effect of removing some of the Auger electrons from their characteristic energy position in an Auger peak and transferringthem to lower energies as part of the “loss tail,” which starts at the low-energy side of the Auger peak and extends all the way to zero energy. The inelastic collision process is characterized by an inelastic mean free path, which is the distance traveled after which only V c of the Auger electrons maintain their initial energy. This is very important because only the electrons that escape the sample with their characteristic Auger energy are u s d in identifying the atoms in 314


Chapter 5

the sample. This process gives the technique its surface specificity. This inelastic mean free path is a function, primarily, of the energy of the electron and, secondarily, of the material through which the electron is traveling. Figure 6 in the X P S article shows many measurements of the inelastic mean free path in various materials and over a wide range of energies, and an estimate of a universal (valid for all materials) inelastic mean free path curve versus energy. The minimum in the mean free path curve, at around 80 eV, is the energy at which electrons travel the shortest distance before suffering an energy-altering scattering event. Thus Auger electrons that happen to have their energy in this vicinity will be those that will have the thinnest sampling depth at the surface. For example, while Si L W Auger electrons from oxidized Si (at approximately 78-ev) are generated at depths ranging from the top monolayer to nearly a pm from a primary electron beam with a typical 5-keV energy, 63% of the electrons that escape without losing any energy come from the top 5 A of the sample. Furthermore, 87% are contributed by the top 10 A of the sample and 95% have been produced in the top 15 A of material. The depth from which there is no longer any signal contribution is ultimately determined by the signal-to-noise ratio in the measured spectrum. If a 5% signal variation is accurately measurable then atoms 3 mean free paths down contribute to the measurement. If 2% of the signal is well above the noise level then atoms at a depth of 4 mean free paths contribute to the measurement. Secondary Electron Collection

As the electrons leave the surface they move in a cosine-shaped intensity distribution away from the analysis point and travel in straight lines until they enter the energy analyzer. The entrance slit of the energy analyzer determines the percentage that are collected, but it is typically just under 20% for the most commonly used energy analyzer, the cylindrical mirror analyzer (CMA). Once in the energy analyzer more electrons are lost by scattering at grids and the CMA transmission is typically 60%. Information in Auger Spectra

Using the best procedures during data acquisition produces spectra with the maximum available information content. Once spectra are recorded that contain the information that is sought using the best procedures for extracting the information from the data is important to maximize the value of the analysis. This section will consider the procedures for data acquisition and the extraction of various types of information available from the data. Data Acquisition

For primarily historical reasons people have come to consider Auger spectra as having the form, aN(E)/dEversus E, where M E ) is the energy distribution of the sec5.3

AES

315

Y-t W

w

E w

z

5 ,

200

I

I

400

600 Electron Volts

Electron Volts

Figure 3

AI

I

I

800

1000

200

I

400 600 Electron Volts

I

800

11

Electron Volts

The ME), dN(E)/d€, dEN(E/d€, and EN(€) forms of secondary electron energy spectra from a slightly contaminated Fe surface.

ondary electrons being detected and E is their energy. This came about because of the properties of various energy analyzers used and because of peculiarities of the analog electronics used to run them. Spectra in this form were acquired by adding an AC component to the energy-selectingvoltage of the energy analyzers (a modulation) and detecting the signal with a lock-in amp1ifier.j This led to the signal being acquired in the differential mode, dN(E)/dEversus E, instead of N(E)versus E. These forms of acquired spectra are shown in Figure 3. With the advent of the CMA and computer-controlled digital signal acquisition, which can be coupled with either pulse counting or voltage-to-frequency conversion for decoupling the signal from the high positive collection voltage, it has finally become practical to discard the modulation and the lock-in amplifier in signal acquisition, as is done in Figure 3(bottom right panel). Acquiring data directly in N(E)(or EXN(E))form, followed by subsequent mathematical processing, provides six valuable advantages: 1 There is an improved signal-to-noise ratio in the raw data. This can be seen in the E X N(E)form of data in Figure 3. 2 The energy analyzer is always operated at its best energy resolution. 3 The measured Auger signal is proportional to the number of atoms sampled. In the derivative mode of data acquisition this is frequently not the case, for example, if an inappropriate modulation voltage is used or if the line shape has 316


Chapter 5

changed due to a change in chemical environment. 4

The physical information in the line shape is immediately available for observation.

5

Peak overlaps can be eliminated simply by peak fitting and subtraction.

6

Loss tail analysis can be applied to the data. (This procedure is discussed below.)

Thus it is best to acquire and store the data in the simplest and least-processed form possible. Extracting Information From the Data

There are at least four kinds of information available from an Auger spectrum. The simplest and by far most frequently used is qualitative information, indicating which elements are present within the sampling volume of the measurement. Next there is quantitative information, which requires a little more care during acquisition to make it extractable, and a little more effort to extract it, but which tells how much of each of the elements is present. Third, there is chemical information which shows the chemical state in which these elements are present. Last, but by far the least used, there is information on the electronic structure of the material, such as the valance-band density of states that is folded into the line shape of transitions involving valance-band electrons. There are considerations to keep in mind in extracting each of these kinds of information. Qualitative Information

Qualitative information can be extracted from Auger spectra quite simply, by a trained eye or by reference to one of the available Auger charts, tables of energies, or handbooks of spectra. The most basic identification is done from the energies of the major peaks in the spectrum. The next level of filtration is done from the peak intensity ratios in the patterns of peaks in the spectra of the elements present. One of the charts ofAuger peak energies available is shown in Figure 4. The useful Auger spectra of the elements fall into groups according to the transition type, KLL, LMM, MNN, etc. If you look across the chart, following a given energy, it is clear that there are many possibilities for intermixing of patterns from different elements, but there are few direct peak overlaps. Generally, if there are peaks from two elements that interfere, there are other peaks from both those elements that do not overlap. One of the most difficult exceptions to this rule is in the case of B and C1: B has only one peak, a KLL peak at 180 eV. C1 has an LMM peak at 180 eV and its JSLL peaks are at 2200-2400 eV, high enough that they are seldom recorded. If there is a real uncertainty as to which of these elements is present, it is necessary to look for the latter peaks. Peak overlaps that totally obscure one of the elements in the spectrum have been shown to be separable.' A Co-Ni alloy film under a Cu film is a combination that produces a spectrum where the Ni peaks are all overlapped by Cu or Co peaks, or 5.3

AES

317

Figure4

One of the numerous available charts of Auger electron energies of the elements.

both. The intensities in the Cu and Co patterns show that another element is present. With the use of background subtraction, standard spectra, and peak fitting and subtraction, the Ni spectrum was uncovered and identified, and even quantitative information, with identified accuracy limitations, was obtained. When listing the elements present from qualitative analysis, the issues of sensitivity and signal-to-noise level arise. The minimum amount of an element that must be present to be detected in an Auger spectrum is a function of a number of variables. Some of these are determined by the element, such as its ionization cross section at the primary energy being used, the Auger yield from its most prolific inner shell vacancy, the energy of its Auger electron (since this determines the elec318


Chapter 5

trons’ mean free path for escape from the solid), etc. Other variables are under the control of the measurement parameters, such as the primary beam energy and current, the energy resolution of the energy analyzer, the angle of incidence of the primary beam onto the sample and the acceptance angle of the energy analyzer. These variables can, to a certain degree, be controlled to yield the maximum signal-tonoise ratio for the element of interest. When these parameters are optimized the detection limit for most elements is on the order of a few times 10’8/cm3 homogeneously distributed, or about 1 atom in 10,000. Quantitative Information

The number of Auger electrons from a particular element emitted from a volume of material under electron bombardment is proportional to the number of atoms of that element in the volume. However it is seldom possible to make a basic, first principles calculation of the concentration of a particular species from an Auger spectrum. Instead, sensitivity factors are used to account for the unknown parameters in the measurement and applied to the signals of all of the species present which are then summed and each divided by the total to calculate the relative atomic percentages present. Of the total number of Auger electrons emitted only a fraction escapes the sample without energy loss. The rest become part of the loss tail on the low-energy side of the Auger peak extending to zero energy and contribute to the background under all of the lower energy Auger peaks in the spectrum. This process must be taken into account when using a sensitivity factor for a particular Auger system. Sensitivity factors are usually taken from pure elemental samples or pure compound samples. This means that the element is homogeneously distributed in the standard. If this is not true in the unknown sample, the percentage of Auger electrons that escape the sample without energy loss changes. If the element is concentrated at the surface, fewer Auger electrons will suffer energy loss; if it is concentrated in a layer beneath another film, more Auger electrons will suffer energy loss before they escape the sample. This can be seen in Figure 5, which shows oxygen in a homogeneous Si02 film, in a surface oxide on Si, and from an Si02 film under a layer of Si. An oxygen sensitivity factor determined from a homogeneous sample would not properly represent the oxygen concentration in the lower two spectra of Figure 5. Sensitivity factors should be measured on the same energy analyzer, at the same energy resolution, at the same primary electron beam energy, and at the same sample orientation to the electron beam and energy analyzer, as the spectra to which they are applied. Only when these precautions are taken can any sort of quantitative accuracy be expected. Even with these precautions the oxygen example discussed above and shown in Figure 5 would present a problem. The most direct way to prevent this problem is by the process referred to above as “loss tail analysis.” This involves comparing the ratios of the peak heights to the loss tail heights, on background subtracted spectra, from the spectrum of the unknown sample and the 5.3

AES

319

Bulk SiOp

Thln Film Si02 on Si

SiOp under Thln Film of Si

450

500

550

KE (eV)

Figure 5

Oxygen spectra from bulk SO2, a thin film of Si02 on Si, and SiO, under a thin film of Si. These spectra have had their backgroundremoved, and so the loss tail can be seen as the height of the spectra at energies below the peaks.

spectrum from which the sensitivity factor was determined. When these ratios are equal the same degree of depth homogeneity of the element in question is assured. Chemical Information

There is a great deal of chemical information in the line shapes and chemical shifts of peaks in Auger spectra. XPS is generally considered to be a more appropriate tool to determine chemistry in a sample. It is true that the photoelectron lines used in XPS are typically narrower and that therefore smaller chemically induced energy shifts can be detected. Moreover, the energy analyzers used in X P S often have better energy resolution. However, it is also true that the chemically induced energy shifts in Auger peaks are usually larger than the corresponding shifts in photoelectron peaks.’ Chemical information is present in Auger spectra in two forms; a shift in the energy of the peak maximum and sometimes as a change in the line shape of the Auger peak. Line shape changes are greatest in transitions involving valance-band electrons, such as the L W transition in Si. Since this line shape is just a weighted 320


Chapter 5

1.2

I

I

I

I

1

48

60

1.o 0.8 c

0.6 I?

0.4

0.2 0.0

0

Figure 6

12

24 36 Depth (arb. units)

Depth profile of an SOz film that had been nitrided by exposure to ammonia. The N and 0 profilesare shown, along with curves of the percentagesof the Si present that is bonded as SiO,, Si3N, "SO," and in S i i i bonds.

self convolution of the valance-band density of states, the line shape varies considerably among spectra originating from Si atoms bound to other Si atoms, Si atoms bound to oxygen atoms, Si atoms bound to nitrogen atoms, etc. The Si KLL spectra are also sensitive to this chemistry, but they manifest it primarily in energy shifts; 7 eV between Si-Si bonding and Si-0 bonding, and smaller shifts from the Si-Si peak for other bonding. The KLL spectra also differ in loss tail heights and in some of the plasma loss peaks that are sometimes present. As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique* and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. Methods for Surface and Thin-Film Characterization

AES analysis is done in one of four modes of analysis. The simplest, most direct, and most often used mode of operation of an Auger spectrometer is the point analysis mode, in which the primary electron beam is positioned on the area of interest on the sample and an Auger survey spectrum is taken. The next most often used mode of analysis is the depth profiling mode. The additional feature in this mode is that an ion beam is directed onto the same area that is being Auger analyzed. The ion beam sputters material off the surface so that the analysis measures the variation, in depth, of the composition of the new surfaces, which are being continu5.3

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ously uncovered. In this mode the Auger data may be acquired in the same, wide energy sweep survey spectrum as in a point analysis, or it may be taken in any number of narrow energy scan windows whose energies are selected to monitor Auger peaks of particular elements of interest. The results shown in Figure 6 above are an example of this mode of analysis, but include additional information on the chemical states of the Si. The third most frequently used mode of analysis is the Auger mapping mode, in which an Auger peak of a particular element is monitored while the primary electron beam is raster scanned over an area. This mode determines the spatial distribution, across the surface, of the element of interest, rather than in depth, as depth profiling does. Of course, the second and third modes can be combined to produce a three-dimensional spatial distribution of the element. The fourth operationalmode is just a subset of the third mode; a line scan of the primary beam is done across a region of interest, instead of rastering over an area. Artifacts That Require Caution

Many artifacts may be present in Auger spectra. Some are caused by the primary electrons interacting with the sample in ways other than creating inner shell vacancies. This can result in removal of species from the surface, through processes like electron-stimulated desorption, or in peaks in the energy distributions that look like Auger peaks but that are photoelectron peaks, ionization loss peaks, or peaks from other processes. Some artifacts are caused by the secondary electrons interacting with the sample on their way out. This can produce peaks due to plasmon excitation processes or can change the detected peak intensities via diffraction processes. During depth profiling, some peaks are caused by the ion-beam interacting with the sample in ways other than simply uniformly removing material. Crystallographic and shadowing effects can produce roughness that increases as depth profiling proceeds and increasingly degrades depth resolution in the profile. Certain ion-beam conditions in combination with certain materials produce ioninduced Auger peaks that can interfere with quantitative accuracy. Variation of sputtering yields among the elements on a surfice can artificiallychange the surface composition. Conclusions

A E S is an important, widely used technique for surface, interface, and thin-film analysis of materials not strongly affected by electron beams. It continues to be improved through advancements in both systems and technique. Higher spatial resolution hardware continues to be developed, along with more rapid data acquisition and processing. Quantitative accuracy is benefiting from improved understanding. Areas of application are expanding through the study of new materials 322


Chapter 5

problems and also by new applications in low-energy electron microscopy and in measurement of surface atom geometries by observing shadowing and diffraction &ts on angular distributions of Auger electrons leavings&s @ED). Related Articles in the Encydopedia

X P S , XPD/AED, SEM, EDS, EPMA, SIMS, and RBS References i

I? Auger. Compt. Rend 177,169,1923; ibid 180,65,1925.

2

J.J. Lander. Pbys. Rev. 91, 1382, 1953. L. A. Harris. J &pi. Pbys. 39,3; ibid 1419, 1968. E. N. Sidiafus. Pbys. Rw.B. 16,1436,1977; ibid. 16,1448,1977; and E. N. Sickahs and C. Kukla. Pbys. Rev. B 19,4056, 1979. M. I? Seah. In Metbodr of.!kfuceAnu&s. (J.M. Walls, Ed.) Cambridge University Press, 1989. Y.E. Strausser, D. Franklin and I? Courtney. Thin SolidFilm. 84, 145, 1981. C. D. Wagner.]. Ehct. Spect. RekztedPben. 10,305, 1977.

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5.4

REELS Reflected Electron Energy-loss Spectroscopy ALBERT J . B E V O L O

Contents

Introduction Basic Principles Common Modes of Analysis and Examples Sample Requirements Artihcts Instrumentation Comparison With Other Techniques Conclusions

Introduction

Reflected Electron Energy-Loss Spectroscopy (REELS) has elemental sensitivities on the order of a few tenths of a percent, phase discrimination at the few-percent level, operator controllable depth resolution from several nm to 0.07 nm, and a lateral resolution as low as 100 nm. REELS can detect any element from hydrogen to uranium and can discriminate between various phases,' such as SnO and Sn02, or diamond and graphite. By varying the primary electron beam energy &, the probing depth can be varied from a minimum of about 0.07 nm to a maximum of 10 nm, where these limits are somewhat sample dependent. The best probing depth is at least twice as good as any other surface technique except ISS, to which it compares favorably with the added advantage of a spatial resolution of a few microns. The lateral resolution is limited only by technological factors that involve producing small electron beam spot sizes at energies below 3 keV, rather than fundamental bearn-solid interac324


Chapter 5

tions like rediffused primary electrons that limit the lateral resolution of SAM, EDS, and SEM techniques.* The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and diffusion as a function of grain boundary orientation. REELS is not nearly as commonly used as AES or X P S . Basic Principles

It is a fundamental principle of quantum mechanics that electrons bound in an atom can have only discrete energy values. Thus, when an electron strikes an atom its electrons can absorb energy from the incident electron in specific, discrete amounts. As a result the scattered incident electron can lose energy only in specific amounts. In EELS an incident electron beam of energy E, bombards an atom or collection of atoms. After the interaction the energy loss Eof the scattered electron beam is measured. Since the electronic energy states of different elements, and of a single element in different chemical environments, are unique, the emitted beam will contain information about the composition and chemistry of the specimen. EELS is an electron-in-electron-outtechnique that has two forms: The emitted electrons can be analyzed after transmission through very thin (5 100 nm) specimens or they can be analyzed after reflection from thick specimens. For samples thinned to 100 nm the transmission mode of EELS yields a lateral resolution of a few nm, but for specimens used in the reflected mode the best lateral resolution (as of this writing) is 100 nm. Transmission electron energy-loss spectra are obtained on STEM or TEM instruments and are covered in Chapter 3. Within the reflected mode there are two major versions distinguished by their energy resolution. The high-energy resolution EELS (HREELS) has a resolution in the meV range, suitable for molecular vibrational excitations and is covered in Chapter 8. The lowenergy resolution reflected EELS (REELS) has a typical resolution of 1 eV, s u a cient ro resolve electronic excitations like plasmons, interband transitions, or corelevel excitations. REELS currently has a lateral resolution of IO0 nm, while HREELS has a resolution in the mm range. HREELS and REELS, because of their high surface sensitivity, require ultrahigh vacuum, while transmission EELS requires only high vacuum. Only REELS and transmission EELS exhibit extended fine structure suitable for atom position determinations. This article considers only REELS. Consider Figure la, which shows the electronic energy states of a solid having broadened valence and conduction bands as well as sharp core-level states X K and Z An incoming electron with energy Eo may excite an electron from any occupied state to any unoccupied state, where the Fermi energy EF separates the two 5.4

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325

EUSTK:PEAU C-WNDUCTlON BAND

-VALENCE

BAND

NE) B.E.

Figure 1

CEELS

VEELS

Representation of a typical density of electron statesfor a metal having X, V. and Zcore levels [top); and REELS spectrum expected from metal shown in top panel (bottom).

types of electronic states: If E 5 EFthen that state is occupied (e.g., core levels or the valence band); if E 1 EF then that state is unoccupied (e.g., conduction-band states). The energy range over which a solid can absorb energy is the convolution of the energy spread of the initial, occupied state with that of the final, unoccupied state. For both interband transitions, defined as valence-to-conduction band excitations and for core-level transitions, defined as core-level-to-conductionband transitions, the final state is the relatively broad conduction band. Since core-level states are narrow, the line shape of the energy-loss spectra afcer a core-level excitation reflects the conduction-band density of states. Each element in the solid, chosen by virtue of the core level involved, can be probed for chemical state information much like AES, except AES probes the occupied valence-band density of states while core-level REELS (CEELS) probes the unoccupied conduction-band density of states. Peaks can occur in CEELS over the whole range of energy below 4.On the other hand, for an interband transition the maximum electron energy loss is given by the energy difference between the bottom of the valence band and the top of the conduction band, which for most materials is 10-40 eV. For metals the minimum energy loss can be as low as zero while for semiconductors and insulators the minimum energy loss is the band gap energy. Since both the initial and final states of an interband transition are involved in chemical bonding, it is expected that the interband REELS spectra will be very sensitive to chemical changes. However, because all states in the conduction and valence bands are strongly mixed, interband transitions cannot be identified easily with a particular element in the solid, as can be done for CEELS. This global character of interband transitions is the same as for valence band X P S , UPS, or optical absorption spectra. Valence electrons also can be excited by interacting with the electron beam to produce a collective, longitudinal charge density oscillation called a plasmon. Plasmons can exist only in solids and liquids, and not in gases because they require electronic states with a strong overlap between atoms. Even insulators can exhibit 326


Chapter 5

plasmons, because plasmons do not require electrons at EF. Plasmon energies range from a few eV to about 35 eV, with most in the range 10-25 eV. In a free-electron metal, the plasmon energy is proportional to the square root of the electron density and so is relatively insensitive to chemical changes. Three-dimensional oscillations within a solid are called bulk plasmons, while two-dimensional oscillations on the surface are called surface plasmons. Suppose a solid with an energy level scheme as in Figure 1 is bombarded by an electron beam of energy 4 where IE;I I4 I IE;I and E; (E;)is the binding energy of the core level X(r). Most of the incident electrons are reflected from the sample surface without energy loss and produce a large peak at 4 called the e h t i c peak. The incident electrons that scatter from the various occupied states form the REELS spectra shown in Figure 1b. Peaks at energy &-E; and 4-4 are due to CEELS excitation, their line shapes reflect the conduction-band density of states. Since the transitions occur in the presence of the empty core level, the line shape in reality reflects the conduction-band density of states in the presence of the core hole. Such a density of states may not be the same as the ground-state density of states that controls the chemical properties of the material, but changes in chemical environment will still result in changes in the excited states. Since the interband and plasmon region involves valence electrons, it is called the valence EELS (VEELS), which with CEELS constitutes a REELS spectrum. Because both plasmons and interband transitions involve valence electrons, sum rules couple their relative intensities and energies in complex ways. If there are sharp, intense peaks in the valence and conduction-band density of states, then the energy of most interband peaks are well defined and very intense. Such is the case for the 3d-, 4d- and 5d-transition metals and the rare earths, with their highly localized d- and f-bands in both valence and conduction bands. Because interband transitions act to dampen the plasmon oscillation they can change the intensity and energy of a plasmon peak if the chemical environment has changed, even if the electron density does not change. Such effects are much less evident for the free electron-like metals, such as AI, Sn and Mg, where VEELS spectra are dominated by the plasmon peaks. An excellent discussion of the effect was given some time ago by C. Powell3 and should be consulted carehlly before interpreting plasmon energy shifts purely on the basis of electron density changes. Common Modes of Analysis and Examples

Perhaps the most common use for REELS is to monitor gas-solid reactions that produce surface films at a total coverage of less than a few monolayers. When 4 is a few hundred eV, the surface sensitivityof REELS is such that over 90% of the signal originates in the topmost monolayer of the sample. A particularly powerful application in this case involves the determination of whether a single phase of variable composition occurs on the top layer or whether islands occur; that is, whether 5.4

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two su&e phases are present simultaneously. In the fbrmer case the plasmon peak of the substrate will remain as one peak but shife in energy with coverage, while in the latter case a new peak from the island phase w ill appear in addition to the substrate peak. The substratepeak will decrease in intensity with increased coverage by the island phase but it will not shift in energy, so that the growth of the island phase can be monitored even if the islands have lateral dimensions much smaller than the incident beam spot size. The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surfice contamination than AES, especially for elements I i Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. Fine structure extending several hundred eV in kinetic energy below a CEELS peak, analogous to EXAFS, have been observed in REELS. Bond lengths of adsorbed species can be determined from Su&ce Electron Energy-Loss Fine Structure (SEELFS)4using a modified EXAFS formalism. Analysis of CEELS5 line shapes often show chemical shifts that have been used to study FeB alloys after recrystallization, C-H bonding in diamondlike films and multiple oxidation states. With the advent of SAM instruments it soon was shown that they could be operated as REELS-mapping microprobes using a technique called Reflected Electron Energy-Loss Microscopy (REELM).The strong VEELS signals can compensatefbr the reduced currents required to maintain & below the pass energy of a CMA, e.g., 3 keV. As a result, maps of very high contrast can be produced in just a few minutes, or maps with a lateral resolution of 100 nm can be p r o d d by M e r reducing the electron current. If 4 is set to a few hundred eV, to optimize the d c e sensitivity, modern S A M instruments can produce spot sizes of a few microns sufficient to generate good REELM images. Figure 2 shows SEM and REELM micrographs of a sample containing ScH2 and Sc(H), the solid solution of hydrogen in scandium. Since SEM ody reveals topographic information and not composition, it is not possible to distinguish between these two phases. A E S cannot distinguish ScH2 from Sc(H). Only VEELS spectra for ScH2 and Sc(H) are diciently different to map the true location of ScHz (dark areas of the REELM image) and Sc(H) (bright areas of the REELM image). REELM is the technique of choice for the detection of metal hydrides in bulk specimens at a lateral resolution of 100 nm. Other applications of REELM include monitoring variations like oxidation, segregation, and hydration in the surface chemistry of polycrystalline naterial~.~’ Differences of 1/ 10 of a monolayer in oxygen coverage due to variations in grain 328


Chapter 5

Figure 2

SEM (left) and REELM (right) micrographs of a hydrogenatedscandium sample. Only the REELM image correctly identifies the scandium solid solution phase (bright) in the presence of the scandium dihydride phase (black).

boundary orientation can be displayed in high-contrast REELM images with a lateral resolution of about 1 pm.

Sample Requirements Samples used in REELS must be ultrahigh-vacuum compatible solids or liquids, but they may be metals, semiconductors or insulators. Because REELS detects a reflected primary electron, rather than a secondary electron like an Auger electron, surface charging will not affect the electron’s detected kinetic energy. As a result, insulating surfaces, even if charged, will generate good REELS signals. To avoid severe charging from the much larger number of secondary electrons it is sufficient to make the flat areas of an insulator about the same size as the incident beam spot size. By adjusting the primary beam energy and angle of incidence, zero absorbed current can be obtained. Artifacts

VEELS spectra are limited in practice to the relatively narrow energy range of about 30 eV over which plasmons or interband transitions can occur. In contrast to AES, XPS, or even CEELS, where excitations can occur over hundreds of eV, the probability of spectral overlap is much higher for VEELS. It is fortunate that most 5.4

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329

REELS spectrometersare in fact Auger spectrometers, so that elemental identifications can be made easily. REELS data are commonly displayed as ME), dN/ dE or second derivatives of ME). The ME) mode has the advantage that the background is not lost, as it is for either of the derivative modes, but the relatively weak CEELS signals are usually dwarfed by the background and so require some level of differentiation to enhance the weak, but sharp, CEELS features. However, the signal-to-noise ratio is degraded by successive differentiations so that the ultimate detectability is worsened. REELS spectra acquired by lock-in detectors can naturally produce either the first- or second-derivativespectra, while those with ME) outputs usually have provisions to mathematically produce the derivativeformat. For the strong VEELS signals, the second derivative has the advantage that the peaks occur at the same energy as they do in the ME) spectra, while those from the first derivative do not. However, a closely spaced, intense pair of ME) excitations will appear as three peaks in the second-derivativemode. It is the author's judgment that the best overall display mode is the first derivative. Not only do the new and old surfaces produce surface plasmons in the islandgrowth mode, but the interface between the growing film and the substrate is also capable of producing an interphase plasmon excitation. Typically an interphase plasmon will appear at an energy intermediate between the surfice plasmons of the two phases. Its intensity will grow as the island phase grows laterally but will eventually disappear as the interface retreats below the thickening island layer. Sometimes it is possible to distinguish surface and bulk plasmons by lowering 4 so that the bulk plasmon will decrease in intensity more rapidly than the surfice plasmon. However both surface states and interband transitions can show the same behavior.

Instrumentation

An Auger spectrometer or scanning Auger microprobe can be operated as a REELS spectrometer or Reflected EELS Microprobe (REELM) instrument at no additional cost in hardware or software. In contrast to AES, REELS requires that the incident electron beam energy 4 be less than the pass energy of the analyzer, usually less than 3 keV. Also, to achieve a reasonable energy resolution, REELS must have 4 less than about 500-1000 eV for the Cylindrical Mirror Analyzers (CMA) typical of most A E S instruments. Incident electron beams with 4 in this range have considerably larger spot sizes or lower currents than those of the 5-20 keV beams used in AES. Electronic processes such as core-level excitations, plasmons, and interband transitions have energy widths of the order of 1 eV. Because deviations from 4 produce chromatic aberrations in the focusing of fine-spot electron 330


Chapter 5

0

I

I

I

50

100

150

Electron Kinetk Energy

Figure 3

200

2

(OW

First-derivativeelectron emission spectra from pure lanthanum taken with primary electron beams having energies of 250 and 235 eV showing the unshifted Auger peaks and the shifted REELS peaks.

beams any beam with a spot size of 100 pm or less is sufficiently monoenergetic for

REELS.~~ Comparison With Other Techniques

In addition to reflected primary electrons there are three other types of emitted electrons: true secondaries,Auger electrons, and back scattered electrons. True secondaries are valence electrons (see Figure 1) emitted into a very intense narrow peak a few eV in kinetic energy, independent of 4 or material composition. They are used to form SEM images that reveal the topography of the surface. Auger electrons are also fixed in energy independent of 4,but occur over a wide energy range that can overlap the CEELS spectrum. An Auger peak and a CEELS peak can be distinguished by changing 4 slightly, say, by 10 eV. Any peak that moves by the same 10 eV must be a CEELS peak and any peak that does not is an Auger peak. This effect is illustrated in Figure 3. Finally, backscattered electrons are all those electrons that are emitted following multiple inelastic collisions, and they form a relatively smooth background that depends on the angle of incidence of the primary beam and the average atomic number Zof the sample, but less so on 4 .

5.4

REELS

331

' Figure 4

x4

VEELS and Auger spectra for tilt angles of 0". 45". and 60" taken from a tin sample covered by a 0.5-nm oxide layer. The doublet AES peaks are the Sn (410) peaks while the singlet AES peak is the 0 (510) taken with the same gain. VEELS peaks are oxide related, while the Sn (410) peak io due primarily to the metallic tin beneaththe oxide, illustratingthe superior depth resolution of VEELS.

and &E are nearly the same, both can be tuned to the In VEELS, because minimum in the inelastic mean free path near 200 eV, and it is then possible to obtain probing depths such that 90% of the signal comes from the top monolayer at high angles of incidence. In AES, % is typically much higher, so the penetration depth of the incident beam is kery large compared to the Auger escape depth. As a result tilting the specimen has little effect and at most 50% of the Auger signal comes from the top monolayer. An example of the superior surface sensitivity of REELS compared to AES is shown in Figure 4, where 4 = 75 eV for the E E L S spectra, and 4 = 3 keV for the A E S spectra. Both sets of first-derivativedata were taken as a function of 0;from a sample of pure tin that had been oxidized to a thickness of 0.50 nm. The two AES spectra at each tilt angle represent the Sn (410) and 0 (5 10) AES spectra. All of the VEELS spectra (even at 0 tilt angle) are dominated by oxide-derived features, while the Sn (410) Auger peak, even at Qi = 60°, is dominated by the metallic substrate. This work is an example of one of the most common uses for REELS, namely, investigations of the very earliest stages of gas-solid interactions. One final note, VEELS was able to distinguish SnO from SnOl because of their different plasmon energies but AES could not because there was no difference in the net core-level energy shift. 332


Chapter 5

Except for hydrogen, both S A M and EDS can be used to map elemental distributiom in addition to REELM. However the role of redifised primary electrons, present in bulk specimens thicker than 100 nm, must be understood. For both SAM and EDS, E, 1 10 keV, so that for a specimen with an atomic number above twenty the incident electrons will diffuse inside the solid over distances of about 1 pm,even if the incident beam is smaller than 1 nm. These electrons can generate X rays within the specimen and Auger electrons at the surface of the specimen over distances of 1 pm. As a result some fraction (about 20 - 50% for AES and about 90% for EDS) of the signal comes from portions of the specimen not directly irradiated by the incident beam. By contrast, VEELS spectra are taken with E, I2 keV and have peaks within 50 eV of E,. Even though the number of back scattered electrons is still high at 2 keV (compared to 10 kev) their lateral distribution is much smdler. More importantly, any back scattered electron with an energy less than 650 eV (which is nearly all of them) cannot produce a VEELS peak within 50 eV of E,. As a result, REELM has a lateral resolution independent of the back scattered electrons, while SAM and EDS have lateral resolutions limited by fundamental beam-solid interactions. As a consequence only the lateral resolution of REELM will benefit from any future reduction in electron beam spot sizes. Conclusions

REELS will continue to be an important surface analytical tool having special features, such as very high surface sensitivityover lateral distances of the order of a few pm and a lateral resolution that is uniquely immune from back scattered electron effects that degrade the lateral resolution of SAM, SEM and EDS. Its universal availability on all types of electron-excited Auger spectrometers is appealing. However in its high-intensity EELS-form spectral overlap problems prevent widespread application of REELS. Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surfice segregation. More frequent use of the element-specific CEELS version of REELM to complement S A M in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low E, with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems.


AES, EDS, HREELS, SEM, TEM, STEM, X P S , EPMA

5.4

REELS

333

References

A. J. Bevolo, J. D. Verhoeven, and M. Noack. Su$ Sei. 134,499, 1983. Comparison of VEELS and AES analysis of the early stages of the oxidation of solid and liquid tin. Illustrates one of the main uses for REELS. 2 A. J. Bevolo. ScanningEhctron Microscopy. 1985, vol. 4, p. 1449. (Scanning Electron Microscopy, Inc. Elk Grove Village, IL) Thorough exposition of the principles and applications of reflected electron energy-loss microscopy (REELM) as well as a comparison to other techniques, such as SAM,EDS and SEM. 3 C. J. Powell. Opt. Soc. Amex 59,738, 1969. Excellent presentation of the interaction between interband and plasmon peaks that is often overlooked in REELS spectral analysis. 4 M. De Crescenzi. Pbys. Rev. Lem. 30,1949,1987. Use of surface electron energy-loss fine structure (SEELFS) to determine oxygen-nickel bond length changes for oxygen absorbed on Ni (100) on a function of coverage from 0 to 1 .O monolayer. 5 I?N. Ross Jr. and K. A. Gaugler. Surf Sei. 122, L579, 1982. Excellent example of chemical state information in CEELS. 6 J. Ghijsen. Surf Sci. 126, 177, 1983. REELS spectra of pure Mg versus primary beam energy showing relative intensities of the bulk and surface plasmons. 7 H. Ibach and J.E. Rowe. Pbys. Rev.89,195 1,1974. Classic example o f 0 2 adsorption Si (1 1 1) and Si (1 00) showing use of VEELS for structure and chemistry analysis. s A. J. Bevolo, M. L. Albers, H. R. Shanks, and J. Shinar.]. AppZ. Pbys. 62, 1240, 1987. VEELS in fixed-spot mode to depth profile hydrogen in amorphous silicon films to determine hydrogen mobility at elevated temperatures. 9 I? Braun. Su$ Sei. 126,714,1983. VEELS study of bulk and surface plasmon energies across Al-Mg alloy phase diagram. 10 B. Schroder. Ninth Confmence on ElectronMicroscopy. 1978, vol. 1, p. 534. (Microscopial Society of Canada, Toronto, Canada) Unique combination of Eo = 30 keV used in HREELS study of a-Si (H) films with meV energy resolution. 1

334


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X-RAY EMISSION TECHNIQUES 6.1 X-Ray Fluorescence, 2CRF 338 6.2 Total Reflection X-Ray FluoresenceAnalysis, TXRF 6.3 Particle-Induced X-Ray Emission, PIXE 357

6.0

349

INTRODUCTION

Three techniques involving the use of X-ray emission to obtain quantitative elemental analysis of materials are described in this chapter. They are X-Ray Fluorescence, XRF, Total Reflection X-Ray Fluorescence,TXRF, and Particle-Induced XRay Emission, PIXE.XRF and TXRF use laboratoryX-ray tubes to excite the emission. PIXE uses high-energy ions from a particle accelerator. The X-ray emission process following the excitation is the same in all three cases, as ir is also for the electron-induced X-ray emission methods (EDS and EMPA) described in Chapter 3. The electron core hole produced by the excitation is filled by an electron falling from a shallower level, the excess energy produced being released as an emitted X ray with a wavelength characteristic of the atomic energy levels involved. Thus elemental identification is provided and quantification can be obtained from intensities. The practical differences between the techniques come from the consequences of using the different excitation sources. In XRF all elements having Z >3 can be detected using WDS (see Chapter 31, though a range of excitation tubes and analyzing crystal monochromators is needed. If EDS (see Chapter 3) is used spectral resolution is much poorer, resulting in overlappingpeaks and a reduced ability to distinguish some elements at low concentrations. Also only Z > 10 can usually be detected. The sample is examined in air, usually with an unfocused X-ray beam without lateral resolution, though special microbeam systems down to 10 pm do exist. For normal usage X-ray penerration will be many microns, resulting in essentially bulk analysis. With special equipment angles of incidence close to grazing can be used, reducing the probing depth considerably (down to 1000 A). Large-area flat samples are then needed. 335

Atomic detection limits are around 0.1% for Z > 15 with a few percent Detection limits for light elements are much poorer since they go roughly as 2 . The quantification algorithms account for several correct ion hctors. XRF has been widely used for solids, powders, and liquids, and more recently for simultaneous determination of both composition and thickness of single and multilayer thin films in the range of a few hundred angstroms to several microns. Such determinations are computer iterative fits to assumed models, however, and so are not necessarily unique. In TXRF the angle of incidence of excitation is reduced to a few mrad. which is below the total reflection angle. Only a few tens of angstroms contribute to the signal under these conditions. The X-ray optics conditions are stringent for this approach, however, and very flat, large-area samples are needed. The method is therefore readily tailored to Si and GaAs wafers, where it is becoming widely used to monitor homogeneous surface impurities at concentrations down to 10'o-lO1' atoms/cm2 for heavy metals and 10'2-10'3 atoms/cm2 for elements such as Si, S, and C1 on GaAs. The method is also usell for thin-film interfaces and multilayers. A variation of the technique is vapor phase decomposition,VPD, where the surface of a Si wafer is dissolved in HF. The resulting solution of impurities is evaporatedin the center of the wafer which is then analyzed by TXRF. The sensitivity can be improved two to three orders of magnitude this way. The cost of TXRF can be up to $600,000 compared to the more modest $50,000 to $300,000 for XRF. PIXE is a n adjunct to RBS (see Chapter 9), using the same partide beam (H or He ions usually) and the same analysis chamber with an added EDS detector. Probing depths for a 2 4 M e V beam (the usual energies) are microns, similar to XRF, but control is possible by going to higher or lower energies. Often there is no lateral resolution but microbeam systems down to a few microns spot size do exist. P K E tends to be used more for surface layer and thin-film analysis than for bulk samples. Its major use has been in the biomedical area, where it has some advantages over XRF in the microbeam mode. Insulators can be a problem because of build up of high charges, leading to discharges. PKE's strength should be its complementary nature to RBS, since it provides unambiguous identification for some elements that are hard to separate by RBS; it is more sensitive to low-Zelements in high-Zmaterials; and it is better at trace analysis in general. RBS,on the other hand, provides a quantitative depth profile of the major constituents, provided they are resolvable. Altogether, however, P K E usage is only about 1% that ofXRF. One should compare capabilities to the electron beam X-ray emission methods of Chapter 3. The major difference is the higher lateral resolution with electron beams and the associated mapping capabilities. Another difhence is the shorter probing depth possible with electrons, except when compared to the specialized TXRF method. Comparing electron-beam EDS to X-ray/particle EDS or electron-beam WDS to X-ray/partide WDS, the electron beams have poorer detection limits because of the greater X-ray background associated with electron 336

X-RAY EMISSION TECHNIQUES

Chapter 6

excitation. The electron-beam methods are always done in vacuum, of course, whereas XRF and TXRF are done in air. XRF in the WDX mode dominates in terms of the number of systems in use in the world (about 10,000). In principle all the X-ray emission methods can give chemical state information fiom small shifts and line shape changes (cE, X P S and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are: the instrumental resolution of commercial systems is not adequate; and the emission lines routinely used for elemental analysis are often not those most useful for chemical shift meas-urements. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission.

337

6.1

XRF X-Ray Fluorescence T l N G C . HUANG

Contents

Introduction Basic Principles Instrumentation Analytical Capabilities Applications Related Techniques Conclusions

Introduction

X-Ray Fluorescence (XRF) is a nondestructive method used for elemental analysis of materials. An X-ray source is used to irradiate the specimen and to cause the elements in the specimen to emit (or fluoresce) their characteristicX rays. A detector system is used to measure the positions of the fluorescent X-ray peaks for qualitative identification of the elements present, and to measure the intensities of the peaks for quantitative determination of the composition. All elements but low-2 elements-H, He, and L i - c a n be routinely analyzed by XRF. Since the 1950s XRF has been used extensively for the analysis of solids, powders, and liquids. The technique was extended to analyze thin-film materials in the 1970s. XRF can be used routinely for the simultaneous determination of elemental composition and thickness of thin films. The technique is nondestructive, rapid, precise, and potentially very accurate. The results are in good agreement with other elemental analysis techniques including wet chemical, electron-beam excitation techniques, etc. 338


Chapter 6

Basic Principles

The hndamental principles of XRF can be found in the literature.1-3 Briefly, X rays are electromagnetic radiation of very high energy (or short wavelength).The unit of measurement for X rays is the angstrom (A),which is equal to lo4 cm.When an Xray photon strikes an atom and knocks out an inner shell electron, if the incident photon has energy greater than the binding energy of the inner shell electron, a readjustment occurs in the atom by filling the inner shell vacancy with one of the outer electrons and simultaneously emitting an X-ray photon. The emitted photon (or fluorescent radiation) has the characteristicenergy of the difference between the binding energies of the inner and the outer shells. The penetration depth of a highenergy photon into a material is normally in the range. (Another method commonly used to produce X rays is electron-beam excitation; the penetration depth of an electron beam is about an order of magnitude smaller than that of X rays. See the articles on EDS and EPMtL) Measurements of the characteristic X-ray line spectra of a number of elements were first reported by H. G. J. Moseley in 1913.He found that the square root of the frequency of the various X-ray lines exhibited a linear relationship with the atomic number of the element emitting the lines. This hdamental “Moseleylaw” shows that each element has a characteristic X-ray spectrum and that the wavelengths vary in a regular fashion form one element to another. The wavelengths decrease as the atomic numbers of the elements increase. In addition to the spectra of pure elements, Moseley obtained the spectrum of brass, which showed strong Cu and weak Z n X-ray lines; this was the first XRF analysis. The use of XRF for routine spectrochemical analysis of materials was not carried out, however, until the introduction of modern X-ray equipment in the late 1940s. Instrumentation

The instrumentation required to carry out XRF measurements normally comprises three major portions: the primary X-ray source, the crystal spectrometer, and the detection system. A schematicX-ray experiment is shown in Figure 1. Fluorescent X rays emitted from the specimen are caused by high-energy (or short-wavelength) incident X rays generated by the X-ray tube. The fluorescentX rays fiom the specimen travel in a certain direction, pass through the primary collimator. The andyzing crystal, oriented to reflect from a set of crystal planes of known dspacing, reflects one X-ray wavelength (A) at a given angle (e) in accordance with Bragg’s law: = 2dsin0, where n is a small positive integer giving the order of reflection. By rotating the analyzing crystal at one-half the angular speed of the detector, the various wavelengths from the fluorescent X rays are reflected one by one as the analyzing crystal makes the proper angle 8 for each wavelength. The intensity of at each wavelength is then recorded by the detector. This procedure is known also as the 6.1

XRF

339

X-ray tube

Analyzing crystal Figure 1

Schematic of XRF experiment.

wavelength-dispersive method. (The wavelength-dispersive method is used extensively in EPMA, see the EPMA article in this volume.) X-RaySoums

A sealed X-ray tube having a W, Cu, Rh,Mo, Ag,or Cr target is commonly used as the primary X-ray source to directly excite the specimen. A secondary target material located outside the X-ray tube is used sometimes to excite fluorescence. This has the advantages of selecting the most efficient energy close to the absorption edge of the element to be analyzed and of reducing (or not exciting) interfering elements. (The intensity is much reduced, however.) X-ray sources, including synchrotron radiation and radioactive isotopes like 55Fe(which emits Mn K X rays) and AM-24 1 (Np L X rays) are used in place of an X-ray tube in some applications. Analyzing Crystals

Crystals commonly used in XRF are: LiF (200) and (220), which have 2Cspacings

of 4.028 and 2.848 A, respectively; pyrolytic graphite (OO2), spacing 6.715 A; PET(OO2), spacing 8.742 A; TAP(OOl), spacing 25.7 A; and synthetic multilayers ofW/Si, W/C, V/C,Ni/C, and Mo/B&, spacing 55-160 A. The lowest-Zelement that can be detected and reflected efficientlydepends on the Cspacing of the analyzing crystal selected. The crystals are usually mosaic, and each reflection is spread over a small angular range. It is thus important that the crystal used be of good quality to obtain intensive and sharp XRF peaks. The angular spread of the 340


Chapter 6

FeKa

60

50 "20

XRF spectrum of MnFe/NiFe thin film.

Figure2

peaks, or the dispersion, de/& = n/(2dcose), increases with decreasing d The dispersion thus can be increased by selecting a crystal with a smaller d X-Ray Detection Systems

The detectors generally used are scintillation counters having thin Be windows and NaI-T1 crystals for short wavelengths (above 3 A or 4 kev), and gas-flow proportional counters having very low absorbing windows and Ar/CH* gas for long wavelengths (below 2 A or 6 kev). A single-channel pulse amplitude analyzer is used to accept fluorescent X rays within a selected wavelength range to improve peak-to-background ratios and to eliminate unwanted high-order reflections. The counting times required for measurement range between a few seconds and several minutes per element, depending on specimen characteristicsand the desired precision. A typical XRF spectrum of a FeMn/NiFe thin film is plotted in Figure 2. The Ka and Kp XRF fluorescent peaks from the fdm are identified, and the remaining peaks are those from the spectrum of the X-ray tube. The experimental conditions included a Mo target X-ray tube operated at 45 kV, a LiF (200) analyzing crystal, and a scintillation counter with a single-channel pulse amplitude analyzer. The energy resolution of the Mn K a peak at 5.89 keV was 24 eV, compared to 145 eV for a Si (Li) solid-state energy-dispersivesystem (see EDS article). The high spectral resolution of the wavelength-dispersive method made possible the measurements of Ni, Fe, and Mn fiee of interference from adjacent peaks. Analytical Capabilities Elemental Depth Profiling

The X-ray penetration depth in a material depends on the angle of incidence. It increases from a few tens of A near the total reflection region to several pm at large 6.1

XRF

341

incidence angles (a few tens of degrees). The XRF beam, which originates from variable depths, can be used for elemental depth analysis. For example, the grazing incidence XRF method has been used for studies of concentration profiles of a dissolved polymer near the air/liquid intehce? Langmuir-Blodgett multilayer^,^ and multiple-layer fdms on substrates.6 This type of analysis requires a parallel-incidence beam geometry, which currently is not possible with a conventional spectrometer. Chemical State Analysii

The XRF wavelengths and relative intensities of a given element are constant to first approximation. Small changes may occur when the distribution of the outer (or valence) electron changes. A major area of research in XRF involves the use of "soft" X-ray emission (or long-wavelength XRF) spectra for chemical state analysis. Soft X-ray peaks often exhibit fine structure, which is a direct indication of the electronic structure (or chemical bonding) around the emitting atom. Thus the shift in peak position, change in intensity distribution, or appearance of additional peaks can be correlated with a variety of chemical factors, including the oxidation state, coordination number, nature of covalently bound ligands, etc. The equipment required for soft X-ray analysis is almost identical to that required for conventional XRF, with one major exception. Since it is a study of transitions involving the outer orbits and therefore long wavelengths, soft X-ray analysis employs a long-wavelength X-ray source such as Al(8.34 A for Al Ka)or Cu (13.36 for Cu La).Special analyzing crystals or gratings for measuring wavelengths in the range 10-1 00 A also are needed.7 QuantitativeAnalysis

In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films. In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc. Two general methods used for making these corrections are the empirical parameters method and the fundamental parameters methods. The empirical parameters method uses simple mathematical approximation equations, whose coefficients (empirical parameters) are predetermined from the experimental intensities and known compositions and thicknesses of thin-film standards. A large number of standards are needed for the predetermination of the empirical parameters before actual analysis of an unknown is possible. Because of the difficulty in obtaining properly calibrated thin-film standards with either the same composition or thickness as the unknown, the use of the empirical parameters method for the routine XRF analysis of thin films is very limited. 342


Chapter 6

The fundamental parameters method uses XRF equations derived directly from first principles. Primary and secondary excitations are taken into account. Primary excitations are caused directly by the incident X rays from the X-ray source, while the secondary excitations are caused by other elements in the same film, whose primary fluorescent X-ray radiation has sufficient energy to excite the characteristic radiation of the anal+ element. Higher order excitations are generallyconsidered insignificant because of their much lower intensities. XRF equations relate intensiry; composition, and thickness through physical constants (fundamental pararneters) like fluorescent yields, atomic transition probabilities, absorption coefficients, etc. For example, the XRF equations for single-layer films were reported by Laguitton and Parrish,' and for multiple-layer films by M a ~ ~ t l eThe r . ~ equations for-thin films are very complex, and the values of composition and thickness cannot be determined directly from the observed intensities. They are obtained by computer iteration using either linear or hyperbolic approximation algorithm. The h d a mental parameters technique is suitable for the analysis of thin films because it requires a minimum number of pure or mixed element and bulk or thin-film standards. Applications

The principle application of XRF thin-film analysis is in the simultaneous determination of composition and thickness. The technique has been used for the routine analysis of single-layer films' since 1977 and multiple-layer filmsio since 1986. Two main sources of publications in the fields are the annual volumes of Advances in X-RayAm&s by Plenum Press, New York, and the Journal of X-RaySpectrometry by Heyden and Sons, London. Typical examples on the analysis of single-layer films and multiple-layer films are used to illustrate the capabilities of the technique. Single-Layer Films

Evaporated FeNi films with a large range of compositions were selected because of the strong absorption of Ni and enhancement of Fe K X rays in the films. XRF compositions of 7 FeNi films deposited on quartz substrates are listed in Table 1 and are compared to those obtained by the Atomic Absorption Spectroscopy (AAS) and the Electron Probe Microanalysis (EPMA). Since the strong X-ray absorption and enhancement effects are severe for both XRF and EPMA but not present in AAS, a comparison between the XRF results and the two non-XRF techniques provide a useful evaluation ofXRF.". As shown in Table 1 ,there is good agreement between results of XRF and AAS or EPMA, and the average deviation is 0.9% between XRF and AAS and is 1.1% between XRF and EPMA. It is worth noting that the compositions of more than half of the 7 FeNi films obtained by XRF, AAS, and EPMA are significantly different fiom the intended compositions (see values inside the parentheses listed in column 1 of Table 1). The discrepancy shows the

'*

6.1

XRF

343

Fe (5)-Ni (95)

4.2

5.0

2.5

Fe (10)-Ni (90)

9.2

9.0

6.2

Fe (20)-Ni (80)

19.4

19.2

19.4

Fe (34)-Ni (66)

47.3

48.4

44.5

Fe (50)-Ni (50)

59.1

61.7

59.1

Fe (66)-Ni (34)

78.9

79.8

78.4

Fe (80)-Ni (20)

89.2

89.6

89.2

Table 1

Fe concentrations 1% wt.) for FoNi films.

risk of using intended composition and the important of determining composition experimentallyby XRF or other reliable techniques. The volume density p and thickness tof a film appear together as a single parameter ptin the XRF equations, the value of pt, the areal density (not the thickness) is determined directly by iteration. From the areal density, the film thickness can be calculated when the volume density is known experimentally or theoretically. Using the volume densities calculated fiom the film composition and the published volume densities of pure elements, the thicknesses of 12 FezoNigo films were calculated from the XRF areal densities and are compared to those obtained by a nonXRF technique (i.e., AAS or a deposition monitor). As shown in Table 2, good agreement between XRF and non-XRF thicknesses are obtained with average and maximum deviations of 2.95% and 6.7%, respectively (see the last column of Table 2). The volume density can also be calculated from the XRF areal density when the thickness of a film is known. For example, the volume densities of 8 Fe19Ni81 permalloy films with known thicknesses of 5O-10,OoO A were calculated from the XRF areal densities. The calculation shows that the volume density of the permalloy is not constant and changes systematicallywith the film’s thickness. It is equal to the bulk value of 8.75 g /cm3 for films of 1000 A or greater thickness, decreases to 94% of the bulk value for the 500-A film, and to 81% for the 50-A tilm.12 Multiple-layer F i h s

XRF analysis of multiple-layer films is very complex because of the presence of XRF absorption and enhancement effects, not only between elements in the same layer but also between all layers in the fdm. Equations fbr the calculation of XRF intensities for multiplelayer films are avaiIabIe from the Iiterature.9’ l3 Proper correc344


Chapter 6

F h

m

1

825

848

2.8

2

858

848

1.2

3

1117

1142

2.2

4

3180

2967

6.7

5

3215

3011

6.3

6

3558

3473

2.4

7

3579

3524

1.5

8

4090

4070

0.5

9

5533

5452

1.5

10

5550

5237

5.6

11

5601

5655

1 .o

12

6283

6053

3.7

Non-XRF'

A/XRF (%)b

a. Either AAS or monitor.

b. A = IXRF - non-XRFI. Table 2

mi-

(A) for wifilms.

tions for intralayer and interlayer effects are essential for a successful XRF analysis of multiple-layer films. The accuracy of XRF compositions and thicknesses for multiple-layer films was bund to be e q d to those for single-layer films. For example, XRF was used successfully to analyze two triple-layer films of Cry l4 The two films, T1 and T2, have Cu, and FeNi deposited on Si substrates.". identical individual CryCu and FeNi layers but different order. In T1,the FeNi layer is on top, the Cu layer in the middle, and the Cr layer at the bottom; in T2, the positions of the Cr and FeNi layers are reversed, with Cr on top and FeNi at the bottom; meanwhile the Cu layer remains in the middle. Because of this reversal of layer order, interlayer absorption and enhancement effects are grossly different between these two films. This led to large differences in the observed intensities between these two films. The differences between T1 and T2 were -17%, +2%, +20%, and +15%, respectively, for the CryCu, Feyand Ni K a observed intensities.12 Using the same set of observed XRF intensities, rhe results obtained by two different analysisprograms: LAMA-I11from the US12and DF270 fiom Japan'* are essentially the same within a relative deviation of 0.2% in composition and 1% in 6.1

XRF

345

T1 (FeNilCulG)

T2 (CrlCdFeNi)

s1

(Cr)

s2 (Mi)

s3 (cu)

Fe (% wt.)

10.25

10.25

-

10.50

-

Ni (% wt.)

89.75

89.75

-

89.50

-

-

-

tc,(4

1652

1698

1674

tFeNi(&

2121

2048

-

tCu@)

2470

2457

-

Table 3

2115

-

2416

XRF mutts for films of Cr, FeNi, and Cu.

thickness. The results obtained by the LAMA-I11 program are listed in Table 3. In spite of the large differences in the observed intensities of Cr, Fe and Ni, the compositions and thickness of all three layers determined by XRF are essentially the same for T1 and T2. For comparison, an XRF analysis was also done on three single-layer Cr, FeNi, and Cu films (S 1, S2, and S3) prepared under identical deposition conditions to the two triplelayer films. As shown in Table 3, good agreement was obtained between the single- and triple-layer films. This indicates that the severe interlayer enhancement and absorption effects observed in T1 and T2 were corrected properly. It is also worth noting that the deviations between the results of the triple- and single-layer films are within the accuracy reported for the single-layer fhs. In multiple-layer thin films, it is possible that some of the elements may be present simultaneously in two or more layers. XRF analysis of this type of f h can be complicated and cannot be made solely from their observed intensities. Additional information, such as the compositions or thickness of some of the layers is needed. The amount of additional non-XRF information required depends on the complexity of the fdm. For example, in the analysis of a FeMn/NiFe double-layer film, the additional information needed can be the composition or thickness of either the FeMn or NiFe layer. Using the composition or thickness of one of the film predetermined from a single-layer film deposited under identical conditions, XRF analysis of the FeMn/NiFe film was ~uccessful.~~ Related Techniques

XRF is closely related to the EPMA, energy-dispersiveX-Ray Spectroscopy (EDS), and total reflection X-Ray Fluorescence (TRXF), which are described elsewhere in this encyclopedia. Brief comparisons between XRF and each of these three techniques are given below. 346


Chapter 6

EPMA

Both XRF and EPMA are used for elemental analysis of thin films. XRF uses a nonfocusing X-ray source, while EPMA uses a focusing electron beam to generate fluorescent X rays. XRF gives information over a large area, up to cm in diameter, while EPMA samples small spots, pm in size. An important use of EPMA is in point-topoint analysis of elemental distribution. Microanalysis on a sub-prn scale can be done with electron microscopes. The penetration depth for an X-ray beam is normally in the 10-~unrange, while it is around 1 ~ITI for an electron beam. There is, therefore, also a difference in the depth of material analyzed by XRF and EPMA. EDS

EDS is another widely used elemental analysis technique and employs a solid state detector with a multichannel analyzer to detect and resolve fluorescent X rays according to their energies. EDS uses either X rays or an electron beam as a source to excite fluorescence. Unlike XRF, which uses the wavelength-dispersive method to record X-ray intensities one by one, EDS collects all the fluorescent X rays from a specimen simultaneously.A limitation of EDS is its energy resolution, which is an order of magnitude poorer than that of the wavelength-dispersive method. For example, the Ka peaks of transition elements overlap the KP peaks of the next lighter element, which cause analytical difficulties. The poorer resolution also causes relatively lower peak-to-background ratios in EDS data. TXRF

XRF at large incident angles, as described in this article, is normally used for elemental analysis of major concentrations of 0.1 % or higher. Total Reflection X-Ray Fluorescence (TXRF) with grazing-incidence angles of a few tenths of a degree is used for trace-element analysis. Detectable limits down to lo9 atoms/cm2 are now attainable using a monochromatic X-ray source. Examples of the use of this technique in d e r technology are given in the article on TXRF in this volume.

Conclusions

XRF is one of the most powerful analysis technique for the elemental-composition and layer-thickness determination of thin-film materials. The technique is nondestructive, inexpensive, rapid, precise and potentially very accurate. XRF characterization of thin films is important for the research, development, and manufacture of electronic, magnetic, optical, semiconducting,superconducting, and other types of high-technology materials. Future development is expected in the area of microbeam XRF, scanning XRF microscopy, grazing-incidenceXRF analysis of surfices and buried interfaces, long-wavelength XRF and chemical state analysis, and synchrotron XRF. 6.1

XRF

347

Related Articles in the Encyclopedla

EPMA, EDS, and TXRF References

L. S . Birks. X-&y SpemchemkdAna~sis. Second Edition, Wiley, New York, 1969.A brief introduction to 2UG, it will be useful to those who are interested in knowing enough about the technique to be able to use it for routine analysis. A separate chapter on EPMA also is included. 2 E. I? Bertin. Principh and Pnactice X-RaySpectrometricAna&s. Plenum, New York, 1970.A practical textbook that also serves as a laboratoryhandbook, although sommhat dated. 3 R Jenkins.An Induction to X-RaySpectrometry. Heyden, London, 1974.A p o d introduction to XRF instrumentation, qualitative and quantitative analyses, and chemical-bonding studies. 4 J. M. Bloch, M. Sansone, E Rondela, D. G. Peiffer, I? Pincus, M. W. Kim, and I? M. Eisenberger. Pbys. Rev. Lett. 54,1039,1985. 5 M. J.Bedzyk, G. M. Bommarito, and J. S. Schidkraut. P b y ~ Rev. Lett. 62, 1376,1989. s D.K.G. de Boer. In Advances in X-RayM y s k (C. S. Barrett et al., Eds.) Plenum, New Yo& 1991,Vol. 34,p. 35. 7 B. L. Henke, J. B. Newkirk, and G. R. Mallet, Eds. Advances in X-Ray Adysis. Plenum, New York, 1970,Vol. 13.The proceedings of the 18th Annual Denver Conference on Applications of X-ray Analysis; the central theme of the Conference was interactionsand applications of low-energy 1

X-rayS.

s D. Laguitton and W. Parrish. Anal Cbm.49,1152,1977. 9 M.Mantler. A d . Chim.Acta. 188,25,1986. i o T.C.Huang and W. Parrish. In Advances in X-RayAna&s. (C. S . Barrett, et al., Eds.) Plenum, New York, 1986,Vol. 29,p. 395. '11 T. C.Huang and W. Parrish. In Advances in X-Ray Amz&sh. (G. J. McCarthy et al., Eds.) Plenum, New York, 1979,Vol. 22, p. 43. 12 T. C. Huang. Thin Solid Films. 157,283,1988; and X-RaySpect. 20,29, 1991. 13 D.K.G. de Boer.X-my Sped. 19,145,1990. 14 Y .Kataoka andT Arai. In Advances in X-hyAmz&s. (C.S. Barrett et al., Eds.) Plenum, New York, 1990,Vol. 33,p. 220. 348


Chapter 6

6.2

TXRF Total Reflection X-Ray Fluorescence Analysis PETER E I C H I N G E R

Contents Introduction Principles of Direct TXRF x-Raysources VPD-TXRF Semiconductor Applications ComparativeTechniques Conclusions

Introduction

X-Ray Fluorescence analysis (XRF) is a well-established instrumental technique for quantitative analysis of the composition of solids. It is basically a bulk evaluation method, its analytical depth being determined by the penetration depth of the impinging X-ray radiation and the escape depth of the characteristic fluorescence quanta. Sensitivitiesin the ppma range are obtained, and the analysis of the emitted radiation is mostly performed using crystal spectrometers, i.e., by wavelength-dispersive spectroscopy.XRF is applied to a wide range of materials, among them metals, alloys, minerals, and ceramics. In the total reflection mode, with the X rays impinging at a grazing angle onto a specular solid surhce, interference between the incident and the reflected X-ray waves limits the excitation depth to a h monatomic layers in which the radiation intensin/ is locally concentrated. Accordingly this surfice sheet, which has a depth of a few nm, is strongly excited, giving rise to an intensive emission of fluorescence quanta. The bulk of the solid is virtually decoupled by the total reflection, leading to a suppression of matrix background fluorescence radiation. The high sensitivity 6.2

TXRF

349

of TXRF for surface impurities is a result of both effects:compression of the X-ray intensity in the surfice sheet, and suppressionof the bulk fluorescencebackground. TXRF is essentially a surf$ce-analyticaltechnique, used to detect trace amounts Until a few years ago, its applicationwas limited of impurities on specular &. to the analysis of liquids that have been pipetted in microliter volumes on flat quartz substrates and allowed to dry. Subsequent TXRF of the droplet residue presents an attractive, multielement analysis with sensitivities down to the pg level. The main applications of this branch of TXRF are in environmental research. In recent years the applicationof TXRF has been expanded to semiconductortechnology, with its stringent demands for surfice purity, especially with respect to heavymetal contamination.' In this application, the semiconductor substrate is directly subjected to TXRF. Detection limits on the order of lo1' m e d atoms per cm2,corlayer, are obtained on silicon & respondingto 10 ppma of a monatomic&s using a monochromatic X-ray source. The following sections focus on the instrumentation and application of TXRF to semiconductor substrates that are usually electrochemically polished and thus provide ideal conditions for TXRF;the wafer's relatively large diameter allows for automatic adjustment of the critical angle. Dedicated wafer surfice analyzers are on the verge of becoming routine monitoring tools in the semiconductorindustry. Principles of Direct TXRF

The primary X-ray beam is directed onto the solid surfice in grazing incidence. The angle of incidence is kept below the critical angle at which t o d reflection occurs. The critical angle is given by -I1 Jne -

Qc = 3.72X10

E

where QC is the critical angle (mrad), 4is the electron density and E is the quantum energy (kev) .The critical angle t ) ~is inversely proportional to the energy of the X-ray quanta, and increases with the square root of the electron density of the solid. For molybdenum &-radiation and a silicon surfice, Cpc is 1.8 m r d . The angular dependence of the fluorescence yield in the neighborhood of the critical angle should be considered in d e l to establish the chemical of surfice impurities, as well as for quantitation in terms of their concentrations (Figure 1). Agglomerated impurities, such as particles or droplet residues, do not participate in the interference phenomenon leading to total reflection; their fluorescence intensity is independent of the angle of incidence below the critical angle, and drops by a fictor of 2 if the critical angle is surpassed due to the disappearance of the reflected component in the exciting beam (nonrtpcCtingimpurities and residues).

350

X-RAY EMISSIONTECHNIQUES

Chapter 6

residue

1

0

I

2

3

angle of incidence (mrad) Figure 1

Experimental curves for the angular dependence of the fluorescence intensity from plated or sputtered submonatomic Ni layers (open triangles), layers produced by the evaporation of a Ni salt solution (opencircles), and the silicon substrate (filledcircles).

On the other hand, impurities that are homogeneously distributed through a submonatomic layer within the s h c e , such as electrochemicallyplated, sputtered, or evaporated atoms, are part of the reflecting surface and their fluorescentyield shows a pronounced dependence on the incidence angle. These reflectingor pkzted impurities exhibit basically the same angular dependence below the critical angle as the matrix fluorescencefrom the bulk silicon, but they peak at the critical angle. The plated-type impurities are most commonly encountered with semiconductor substrates; they originate, for example, fiom wet chemical processing steps. It is apparent from Figure 1 that a precise control of the angle of incidence is an essential feature of TXRF instrumentation. X-Ray Sources

Sealed conventional fine structure tubes with Mo,W, Cu, or Cr anodes are used as primary X-ray sources, as well as rotating anode tubes, or synchrotron radiation. The maximum energy of the X-ray quanta determines the range of elements acces6.2

TXRF

351

N

9

\

m E

U

a

Atomic Number Figure 2

-ion l i m b obtainedwith Cu and Mo anodes in conjunctionwith a monochromator.2

sible for analysis and the detection limits of the respective elements, as shown in Figure 2 for monochromatized radiation from a Cu and Mo anode. In this example, Fe (Z= 26) can be detected at a level below 10" atoms/cm2 using the Cu anode, but Cu is not detectable. In modern TXRF instrumentation, the primary radiation from the X-ray tube is filtered or monochromatized to reduce the background originating primarily from bremsstrahlung quanta with higher energy than the main characteristicline for the anode material. The higher energy radiation does not fulfill the critical angle condition for total reflection and penetrates into the substrate, thus adding scattered radiation. Energy filtering is achieved using multilayer interference or crystal diffraction. VPD-TXRF

T h e term direct ZWFrefers to surfice impurity analysii with no s u k preparation, as described above, achieving detection limits of 10'o-lO1l cm- for heavymetal atoms on the silicon d c e . The increasing complexityof integmd circuits fabricated fiom silicon wapfs will demand even greater&s purity in the hture, with accordingly betm detection limits in analytical techniques. Detection limits of less than lo9 cm-*can be achieved, fbr example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). The VPD method originallywas developed to determine m d trace impurities on thermally oxidized or bare silicon surfaces in combination with atomic absorp352


Chapter 6

alve

oled water flow Figure3

hot water flow

Schematic arrangement for vapor phase decomposition IVPD) applied to silicon wafers.

tion spectroscopy (VPD-AAS). The silicon wafer is exposed to the vapor of hydrofluoric acid, which dissolves the Si02 surfice layer (native or thermal oxide) according to the reaction:

SO,+ 6HF + H,SiF6

+ 2H,O

(2)

The impurities on the surfice are contained in the resulting water droplet or moisture film, and are collected in situ for h t h e r investigation by scanning the surface with an auxiliary water droplet (e.g., 50 d).The VPD residue is allowed to dry in the center of the wafer and subjected to TXRF analysis. A schematic of a VPD reactor is shown in Figure 3. With VPD preconcentration, the angular dependence of the impurity fluorescence yield follows the curve for residue impurities, as shown in Figure 1, in contrast to the plated-impurity case using direct TXRF. The sensirivity enhancement achieved by VPD is determined by the ratio of :he substrate area to the area of the detector aperture (analyzed area), provided there is full collection of the impurities. This has been demonstrated for Fe and Zn. For Cu and Au, however, only a small percentage can be collected using this techniq~e,~ due to electrochemical plating. An example comparing direct TXRF with VPDTXRF on the same substrate is shown in Figure 4. Semiconductor Applications

In silicon integrated circuit technology, TXRF analysis is applied as a diagnostic tool for heavy-metal contamination in a variety of process steps, including incoming wafer control, preoxidation cleaning, and dry processing equipment evaluation. As an example, Figure 5 shows the effect of applying a standard cleaning to silicon 6.2

TXRF

353

0.00

Figure 4

Energy, keV

20.48

Direct TXRF (upper spectrum, recording time 3000 s) and VPD-TXRF (lower spectrum, recording time 300 s) on a silicon wafer surface. The sensitivity enhancement for Zn and Fe is two orders of magnitude. The measurements were made with a nonmonochromatizedinstrument.

wafers received from a commercial vendor: The contamination is similar for wafers 1,2, and 3, with K, Ca, Fe, and Zn as the predominant metal impurities in concentrations of 1011-1012atoms/cm2. Cleaning on wafers 4, 5 , and 6 removes all metals to a level of less than 10" cm-2, except for Fe which is still detectable. The deposition of Br is due to the cleaning solution and is not considered harmful. The analysis has been performed using VPD-TXRF. With gallium arsenide, additional elements, such as Si, S, and C1, are of interest because of their doping character. Impurity levels on the order of 10l2 cm-2 are encountered with commercial substrates, which can be readily assessed using direct TXRF.* VPD-TXRF is not possible in this case because of the lack of a native oxide layer on gallium arsenide. 354


Chapter 6

as received

D I l Br D m c1 hz9 Zn Fe

Ti 0 Sr 5 0 Ca

m

K

after cleaning

Figure 5

Effect of cleaning on the surface purity of silicon wafers, as measuredby VPDTXRF.

ComparativeTechniques

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF,compared to VPDAAS,is its multielement capability; AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank d u e s is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the su&ce. In addition, TXRF is fast,with an typical analysis time of 1000s; turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 6.2

TXRF

355

Conclusions Triggered by the purity demands of silicon integrated circuit technology, TXRF has seen a rapid development in its application to solid surfaces during the last seven years, which is reflected in the availability of a variety of commercial instruments and services today. The investigation of surface cleanliness, however, does not exhaust the inherent capabilities of TXRF: From the detailed angular dependence of the fluorescence yields around the critical angle for total reflection, information may be obtained about thin frlms, interfaces, or multilayer structures in the hture. An overview of these trends can be found in Proceedings of the Intpmational It is also possible, in principle, to Worhhop on TotalRL$ectionX-RayFlaore~cence.~ obtain chemical state information, along with elemental analysis. This requires the use of high-energy resolution techniques to detect small shiftsin line positions in the emitted fluorescence. In the soft X-ray region, instrumentation of this type is not commercially available. Related Articles in the Encydopedia

XRFandNAA References 1

2

3

4

5

I?Eichinger, H. J. Rath, and H. Schwenke. In: SemiconductorFabrication: Technology and Metroha ASTM STP 990. (D. C. Gupta, ed.) American Society for Testing and Materials, 305, 1989. U. Weisbrod, R. Gutschke, J. JSnoth, and H. Schwenke. FreseniusJ.A d Cbem. 1991,in press. C. Neumann and I?Eichinger. SpectrochimicaActa BAt. Sped. 46, Vol. 10, 1369, 1991. R S. Hockett, J. Men, and J. I? Tower. Proceedings of the Fiph Confienee on Semi-InsulatingIII-VMat&h. Toronto, 1990. Proceedings of the International Workshop on Total &$ection X-RayFlmrescence. Vienna, 1990 (same as Reference 3).


Chapter 6

PIXE

6.3

Particle-InducedX-Ray Emission RONALD G . MUSKET

Contents

Introduction Basic Principles Modes of Analysis Quantification Artifacts Conclusions

Introduction

Parride-Induced X-Rayemission (PIXE) is a quantitative, nondestructive analysis technique that relies on the spectrometry of characteristic X rays emitted when high-energy particles (4.3-10 MeV) ionize atoms of a specimen. PIXE provides simultaneous analysis of many elements with sensitivity and detection limits that compare very favorably with other techniques. Since the first quantitative measurePIXE ~ has been applied successfullyin a ments of thin metal films in the late 1 9 6 0 ~ variety of fields, including corrosion and oxidation, semiconductors, metallurgy, thin films, geoscience, air pollution and atmospheric science, biology, medicine, art, archaeology, water analysis, and forensic science. During this 25-year period, PIXE has matured and developed into a routine analytical tool for many applications. A recent survey of over a hundred PIXE systems throughout the world revealed that the main areas of application are currently biomedicine (a major application for 40% of the systems), materials (30%) and aerosols (20%).' A detailed discussion of P E E is presented in the recent book by Johansson and Campbell: which was a major reference for this article. 6.3

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357

Generally the particles used for PIXE are protons and helium ions. PIXE is one of three techniques that rely on the spectrometry of X rays emitted during irradiation of a specimen. The other techniques use irradiation by electrons (electron microprobe analysis, EMPA, and energy-dispersive X rays, EDS) and photons (X-Ray Fluorescence, XRF). In principle, each of these techniques can be used to analyze simultaneously for a large range of elementefrom lithium to uranium. For simultaneous, multi-elemental determinations using a standard energy-dispersive, X-ray spectromerer, the range of elements is reduced to those with atomic number Z > 11. Analysis for elements with Z > 5 can be performed with windowless or high transmission-windowed detectors. Wavelength-dispersive detection systems can be used for high-resolution X-ray spectrometry of,at most, a few elements at a time; however, the improved resolution yields information on the chemical bonding of the element monitored. In this article only the results from the widely used lithium-drifted, silicon-Si(Li)-energy-dispersive spectrometers will be discussed. (See also the article on EDS.) Compared to EDS, which uses 10-100 keV electrons, PIXE provides orders-ofmagnitude improvement in the detection limits for trace elements. This is a consequence of the much reduced background associated with the deceleration of ions (called bremrstrahung3 compared to that generated by the stopping of the electrons, and of the similarity of the cross sections for ionizing atoms by ions and electrons. Detailed comparison of PIXE with XRF showed that PIXE should be preferred for the analysis of thin samples, surface layers, and samples with limited amounts of material^.^ XRF is better for bulk analysis and thick specimens because the somewhat shallow penetration of the ions (e.g., tens of pm for protons) limits the analytical volume in PIXE. Basic Principles

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatteringswith the electrons of the material, and it's energy is continuously reduced as a function of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizing an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. Interactions of Ions With Materials

After monoenergetic protons and helium ions having energies between about 0.3 and 10 MeV enter a material, they begin slowing down by inelastically scattering 358


Chapter 6

with electrons and elasticallyscattering with atomic nuclei. The statistical nature of this slowing process leads to a distribution of implanted ions about a mean depth called the projected range Rp, which has a standard deviation ARp. These losses and ranges can be evaluated for various combinations of incident ion and target material using well-developed calculational procedures, such as the Monte Carlo code called TRIM (transport of ions in materials)! When the velocity of the ions is much greater than that of the bound electrons, interactions with the electrons dominate and the ion path can be considered to be a straight line. At any depth associated with the straight-line part of the trajectory, the number of ions is preserved because only about one ion in a million is backscattered; however, their energies decrease slowly and spread increasingly about the average as a result of interactions with the electrons. This energy regime corresponds to that of the dominant X-ray production cross sections; thus modeling the source term for X rays is much simpler fbr ions than for electrons, which undergo strong deviations from their initial flight path as a result of collisions with the electrons of the target. X-Ray Production

Although there have been various theoretical schemes for calculating cross sections for inner-shell ionizations by protons and helium ions, many PIXE workers now use the K and L-shell cross sections calculated using the ECPSSR method. This method involves a series of modifications to the plane-wave Born approximation, which uses perturbation theory to describe the transition from an initial state consisting of a plane-wave projectile and a bound atomic electron to a final state consisting of a plane-wave particle and an ejected continuum electron. The ECPSSR method includes the deflection and velocity changes of the projectile caused by energy losses, the Coulomb field of the target nucleus, perturbation of the atomic stationary states, and relativistic effect^.^ A tabulation6 of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescenceyields, are displayed in Figure 1. Although these Kshell cross sections have been found to agree with available experimental values within lo%, which is adequate for standardless PIXE, the accuracy of the L-shell cross sections is limited mainly by the uncertainties in the various Lshell fluorescenceyields. Knowledge of these yields is necessary to convert X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The M-shell situation is even more complicated. The production of characteristic X rays is determined by the cross sections discussed above, but the observed X-ray spectra include both these characteristicpeaks and a continuous background radiation. A detailed investigation of the origin of 6.3

PIXE

359

Z=8,0

0.0 Figure 1

1.0

20 3.0 ~ensrsVFlev)

40

Calculated KX-ray production cross sections for protons using the tabulated ECPSSR ionization cross sections of Cohen and Harrigan! and the fluorsscence yields calculated as in Johansson et al? (1barn crn2).

le

this background radiation has shown that the dominant source is the bremsstrahlung radiation emitted by the energetic electrons ejected by the The contributions to the background from electron and proton bremsstrahlung radiation caused by 3-MeV protons are shown in Figure 2. Deviations of the experimental results from the calculated curves for X-ray energies above 10 keV probably represent the effects of Compton scattering of y rays fiom excited nuclear states, which were not accounted for in the calculations. In a classical sense, the maximum energy T, that a 3-MeV proton can transfer to a free electron is 6.5 keV. Thus, in Figure 2 the bremsstrahlung radiation is most intense below T, and decreases rapidly at higher energies. From an analytical point of view, this discussion implies that changing the ion energy will not improve the characteristic-to-background (C/B) ratio for X rays having energies below about T, because both the dominant bremsstrahlung background and the characteristic X rays result fiom essentially the same ionization processes. However, reducing the ion energy will shift the electron bremsstrahlung radiation to lower energies and have the effect of improving the C/B ratio (i.e., improving the detection limit) for X-ray peaks at energies above T,. Many P E E workers prefer 2-3 MeV protons because they provide a reasonable compromise between the characteristicX-ray production rate ,and the C/B ratio, while limiting the level of background from nuclear reactions. In hct, most modern ion accelerators used for materials analysis can provide protons with maximum energies of 24 MeV. Detection Limits

In PIXE the X-ray spectrum represents the integral of X-ray production along the path length of the decelerating ion, as mediated by X-ray absorption in the mate360


Chapter 6

tor 3 MeV protoeson A1

----

..DrCnnt

.c W 0 I; W

lo-' El

Figure 2

in KeV

'

2

'

" . " I

5

10

20

50

Er in KeV

Experimental and calculated backgroundradiation production cross sections for (a1360 pg/cm2 plastic foil and (b) 200 p . g / c d AI foil?

rial. Consequently, it is convenient to consider trace analysis for three different cases: thin, free-standing specimens; surface layers (e.g., oxides or coatings) on thick specimens; and thick or bulk specimens. Specimens are considered thin if an ion loses an insignificant amount of energy during its passage through the foil and ifX-ray absorption by the specimen may be neglected. Under these circumstances, the yield of the characteristicX rays can be determined using the ionization cross sections for the energy of the incident ion, and detailed knowledge of the complete composition of the specimen is not needed to make corrections for the particle's energy loss or the absorption of X rays. As shown in Figure 3,' the detection limits for various elements in thin specimens depends on the host matrix. About 0.1 weight part per million (wppm) of elements with atomic numbers near 35 and near 80 can be detected in carbon. Thus, less than g could be detected in or on a 100 pg/cm2 carbon foil using a 1-mm2 beam of 3-MeV protons. The detection of impurities or surface layers (e.g., oxides) on thick specimens is a special situation. Although the X-ray production and absorption assumptions used for thin specimens apply, the X-ray spectra are complicated by the background and characteristicX rays generated in the thick specimen. Consequently, the absolute detection limits are not as good as those given above for thin specimens. However, the detection limits compare very favorably with other surface analysis techniques, and the results can be quantified easily. To date there has not been any systematic study of the detection limits for elements on surfaces; however, representative studies have shown that detectable surhce concentrations for carbon and 6.3

PlXE

361

I

n-n I w

1

20

I

I

I

30

I

I

I

I

I

I

I

I

1

I

I

I

I

40

so

60

70

eo

90

2 of Trace Element Figure3

Calculated detection limits for trace elements in 1 mg/cm2 specimens of carbon,aluminum, and calcium (100 pC of 3-MeV protons)? The dashed curves represent the detection limits if the background radiation is due only to secondary electron bremsstrahlung.

oxygen are about 100 ng C / m 2 on iron using 5-MeV He9 and 30 ng O/cm2 on beryllium using 2-MeV He. lo In thick specimens, the particles ionize atoms along essentially their entire path in the specimen, and calculation of the characteristic X-ray production requires integration of energy-dependent cross sections over all ion energies from the inddent energy to 0 and correction for the absorption of the X rays. Detection limits have been estimated for thick targets when the characteristic KX-ray signal occurs at an energy greater than the bremsstrahlung background (Figure 4). For thick targets,*' limits below 100 ppm are achievablefor elements with Z > 20 in most matrices and can be below 1 ppm for elements near Z = 35 in low-Z matrices; for elements with Z c 20, the limits are no better than 100 ppm in most matrices, but can be considerably better in low-Zmatrices. For example, a detection limit of 10 ppm for oxygen in beryllium has been demonstrated. lo Modes of Analysis Thin, Free-Standing Specimens

Whenever the appropriate specimens can be prepared, this mode is normally the one preferred for trace-element analysis in geoscience, air pollution and atmospheric science, biology, medicine, water analysis, and forensic science. In this case, the ions pass through the specimen with negligible energy loss and there is minimal absorption of X rays. Surface Layers on Bulk Specimens

Included in this dass of thin surhce films are oxides, corrosion, contamination, and deposited layers. Although the presence of the bulk specimen results in increased 362


Chapter 6

. . . . . . . . . . .

I

Concentration giving a peak to background ratio 1 for pmtm

3 MeV p on AI

2 MeV p on AI

c

e

c

jrj5

al C Peak l o bockground mlio 1 for K X-rays from trace element

0

m'

10

20

'

E

30

LO

'

50

8

Z of

7OlO

n

B

n

m

'

'

20

30

LO

50

60

10

trace element

Calculatedminimum concentrationof a trace element in thin and thick carbon and aluminum specimens for 2- and 3-MeV protons!. The dashed curves show the effects of X-ray absorption on detection limits for thick targets (takeoff angle of 45").

Figure 4

background radiation compared to that for thin, free standing specimens, the detzction limits can be sufficiently low to permit calibration of "true" surface analysis techniques, such as Auger electron spectroscopy (AES).In this sense, PIXE fills the "quantitative gap" that exists in surfice and thin-film analysis. As an example, Figure 512 shows the helium-induced X-ray spectrum for an anodized tantalum specimen typical of those used to define the sputtering conditions for AES depth profiling experiments. This represents the simplest surface-layer case because the element of interest in the surfice layer (i.e., oxygen) is not measurable in the bulk. Thus, the 0-K X-ray signal can be directly related to the surface concentration of the oxygen. Bulk Material

Although XRF is generally the X-ray spectrometry method of choice for analysis of major and trace elements in bulk specimens, useful PIXE measurements can be made. A detailed review of the main considerations for thick-target PKE'I provides guidance for trace analysis with known and unknown matrices and bulk analysis when the constituents are unknown. Campbell and Cookson'' also discuss the increased importance of secondary fluorescence and geometrical accuracy for bulk measurements. 6.3

PIXE

363

. . . TafM$ 1 .L'. :.-2-d a

.

: ZL

(1190 A Ta,O,,

18.7 pg O/cm2) 4.0

OO X-ray energy (keW

Figure 5

Helium-ion induced X-ray spectrum from anodized tantalum (fluence 1.5 X 1015He+/cm2).'2

Depth Profiling of Surface Layen on Specimens

Rutherford Backscattering (RBS) provides quantitative, nondestructive elemental depth profiles with depth resolutions sufficient to satisfy many requirements; however, it is generally restricted to the analysis of elements heavier than those in the substrate. The major reason for considering depth profding using PIXE is to remove this restrictive condition and provide quantitative, nondestructive depth profdes for all elements yielding detectable characteristicX rays (i.e., Z>5 for Si(Li) detectors). Because a PIXE specvum represents the integral of all the X rays created along the particle's path, a single PIXE measurement does not provide any depth profile information. All attempts to obtain general depth profdes using PIXE have involved multiple measurements that varied either the beam energy or the angle between the beam and the target, and have compared the results to those calculated for assumed elemental distributions. Profdes measured in a few special cases suggest that the depth resolution by nondestructive PIXE is only about 100 nm and that the absolute concentration values can have errors of 10-50%. Although depth profiling using nondestructive PIXE does not appear promising, PIXE in combination with RBS or with low-energy ion sputtering offers the possibility of quantitative depth profdes for elements with 2 > 5. Because X rays and backscattered particles emanate from the specimen during ion irradiation, both should be detected. In fkt, simultaneously performing PIXE and RBS eliminates ambiguities in the interpretation and modeling of the RBS results.12As an example, consider RBS results from high-Zmaterials with surface layers containing low-Z elements. In such cases, the RBS spectra are dominated by scattering from the high364


Chapter 6

Zelements, with some indication of compositional changes with depth; PIXE provides unequivocal identification of the elements present and, with appropriate calibrations, the absolute areal densities of these elements. Although not commonly available, the combination of PIXE and low-energy ion sputtering can provide quantitative, destructive depth profiles. PME measurements taken after each sputtering period give the quantity of each element remaining and, by difference, the number of atoms (of each element) removed during the sputtering period. Consequently, this combination can yield both the elemental concentrations and the conversion of the sputtering time or fluence to a depth scale. The achievable depth resolution would be determined ultimately by the precision of the PIXE results or nonuniform sputtering effects and could be about 1 nm at the surfaces of specimens. Other Modes of Analysis

For the preceding modes, the discussion implicitly assumed the “normal” conditions for PIXE analysis (ie., few mm-diameter beam, approximately constant beam current, and specimen in vacuum), and ignored the crystallographic nature of the specimen. However, some of the most interesting PIXE results have been obtained using other modes. Reducing the ion beam to a small spot (1-10 pm) and scanning this microbeam across the specimen yields lateral concentration maps analogous to those obtainable with an electron microprobe, but with the advantages inherent in particle-induced spectra and with the potential of much higher spatial resolution, because the particle beam spot and the X-ray source volume have essentially the same lateral dimensions due to the nearly straight-line trajectory of the particles over their X-ray producing range. External-beam PIXE refers to measurements with the specimen removed from a vacuum environment. This mode permits the analysis of large or volatile specimens and consists of allowing the particle beam to exit, through a thin window, the vacuum of the beam line and impinge on the specimen held at atmosphericpressure of air or other gases (e.g., helium). The combination of PIXE with channeling of ions through the open directions (i.e., axial or planar channels) of monocrystalline materials can be. used to determine the location of impurity atoms in either interstitial or substitutional sites or to assess the extent of lattice imperfections in the near-surface region. Channeling measurements are usually performed with RBS, but PIXE offers distinct advantages over FU3S. First, because the PIXE cross sections are larger, the measurements can be performed with lower beam intensities or fluences. In addition, PIXE can be used to locate light elements in a matrix of heavy elements. On-demand beam pulsing has been shown to be effective for eliminating pulse pileup in the X-ray detection system, minimizing the energy dissipated in delicate specimens, yet maximizing the data throughput of the overall system. In essence, 6.3

PIXE

365

the on-demand pulsing system consists of deflecting the beam off the specimen when the detector's amplifying system begins to process a pulse and returning the beam to the specimen at the end of the pulse-processing time. l3 Quantification

Quantification of raw PIXE spectra involves identifying all the peaks, determining the net counts under each peak, and correcting for the energy-dependence of the ionization cross sections as a h n a i o n of depth in the material; the absorption of the X rays in the material; the production of secondary fluorescence;and the fraction of the X rays detected. In general, high accuracy in PIXE is achieved only for thin or homogeneous, thick specimens. Although PUCE spectra are routinely analyzed using least-squares fitting codes, the accuracies are ultimately limited by the accuracies of the fundamental data bases for ionization cross sections, fluorescence yields, ion stopping cross sections, and X-ray absorption effects, and by the calibration procedures used to determine the energy dependence of the fraction of the X rays detected. A detailed comparison of the analysis of the same set of PIXE data using five different spectral fitting programs yielded remarkably good agreement for the results ~btained.'~ The five least-squares fitting codes were used on PIXE spectra from a set of thin biological, environmental, and geological specimens. The results were the same within 5%, which suggests that PIXE analysis has matured to quite an acceptable level. Similar accuracies have been found for thick, homogeneous, complex specimens when corrections for secondary excitation are also included. With appropriate standards, total accuracies of 2% have been demonstrated. Because the determination of the lighter elements (i.e., 5 e Ze 15) are more sensitive to the uncertainties in the data base items listed above, less accuracyshould be expected for these elements. Artifacts

The major artifacts contributing to uncertainties in PIXE results stem from effects caused by bombardment of nonideal specimens, particularly thick specimens. The ideal thick specimen would be a homogeneous, smooth electrical conductor that does not change during bombardment. Except for rather simple, well-defined layered structures (e.g., surface oxide layers), specimens having compositional variations with depth yield spectra whose analyses can have large inaccuracies. Changes in the composition of a specimen over the analyzed depth can be caused by beam heating or by beam charging of the specimen. Beam heating can lead to selective vaporization of some elements or diffusional redistribution of the elements. If the surface charges up to some potential, then electric-field enhanced diffusion can selectively redistribute certain elements. Beam-heating effects usually 366


Chapter 6

can be mitigated by lowering beam intensities or by cooling the specimen. Electricfield enhanced diffusion can be controlled using the procedures described below for eliminatingspecimen charging by the beam. Ion bombardment of electrically insulating specimens can lead to a surface charge giving a potential at the beam spot that can be significant (e.g., from a few kV to > 10 kv). Such potentials can cause s d c e discharges between the spot and the closest grounded conductor. The P E E spectra would then contain characteristic and bremsstrahlung X rays excited by electrons participating in the discharges. Thus, the accuracy for the characteristicpeaks would be reduced, and the relatively large bremsstrahlung radiation would hide the characteristicX-ray peaks of some trace elements. In addition, surhce charging precludes accurate current integration fiom the specimen current, even when electron suppression is used. Standard procedures to eliminate or minimize these effects indude neutralizing the surfice with electrons from a hot filament or a thin metal or carbon foil placed in front of the specimen, but out of view of the X-ray detector; coating the specimen with a thin, conducting layer that either covers the bombarded spot or encircles the spot and leads to a conductor; and introducing a partial pressure of a gas (e.g., helium) into the chamber and letting ionization of the gas by the beam provide a conducting path to discharge the specimen. The geometrical arrangement of the beam axis, the specimen normal, and the detection angle must be well known to obtain accurate P E E results. A rough surhce affects both the ion range and the X-ray absorption. The impact on accuracy will depend on the element of interest and the matrix. The largest effects occur for situations involving high X-ray absorption coeficients. As an example, yield variations of about 25% for sulfur in iron have been calculated fbr 10-pm grooves, when the groove direction is perpendicular to the detector axis.15 Of course, minimal effects are expected when the detector views the specimen along the grooves. Conclusions

Within the last 5-10 years PIXE,using protons and helium ions, has matured into a well-developed analysis technique with a variety of modes of operation. P E E can provide quantitative, nondestructive, and fast analysis of essentially all elements. It is an ideal complement to other techniques (e.g., Rutherford backscattering) that are based on the spectroscopy of particles emitted during the interaction of MeV ion beams with the surfice regions of materials, because i X rays also are emitted from the bombarded specimen. 2 Characteristic X rays provide an unambiguous identification of the elements present. 3 Quantitative analysis of low-Zelements on or in high-Zmaterials can be performed.

6.3

PIXE

367

P E E detection limits for sur& layers on bulk specimens are sufficiently low to permit calibration of true surface analysis techniques (e.g., Auger electron spectroscopy). Development of several existing trends can be expected to enhance the value of P E E in the future. Analyses for low-Zelements (e+ 5 < Z < 15) should receive increased attention because of the availability of Si(Li) detectors with windows that transmit a large fraction of < 1-keV X, rays yet support atmospheric pressure. Appropriate modifications to the existing spectral fitting programs will be required to make quantification for low-Zelements a routine procedure. Since micro-PIXE equipment has just recently become available commercially, a proliferation of micro-PEE capabilities can be anticipated. The demonstrated compatibility and usefdness of simultaneous P E E and RBS should lead to routine applications of such measurements. With specific regard to s u r f i c e analysis, arrangements combining P E E with low-energy sputtering will allow quantitative, high-resolution depth profiling of materials. In addition, combinations of PIXE with true surf$ce analysis techniques (e.g., A E S and X P S ) will improve greatly the quantitative accuracy of these techniques. This work was performed under the auspices of the US DOE by Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. Related Articles in the EncvcropeCria

RBS, ERS, XRF, EDS, and EPMA References 1

T A. Cahill and J. Miranda, private communication, 1991.

z S. A. E. Johansson and J. L. Campbell. PZZ: A Novel Technique,& EkmentaZAna&s.John Wiley, Chichester, 1988. A detailed introduction to PIXE and the relevant literature. 3 J. €? Willis. Nut! Ins& Metb. Pbys. Res. B35,378,1988. Acritid comparison of P E E with XRF. 4

5

6

7

J. F. Ziegler,J. I? Biersack, and U. Littmark. TheStoppingandRangeofIons in Soidr. Pergamon Press, New York, 1985. Source book for TRIM calculations. W. Brandt and G. Lapicki. Pbys. Rev. A20,465,1979; A23,1717,1981. Original papers on ECPSSR calculations of cross sections. D. D. Cohen and M. Harrigan. Atomic Data andNuckarData Zbks. 33, 255,1985. Tabulated ECPSSR cross sections for protons and He+. F. Folkmann, C. Gaarde, T. HUUS,and K. Kemp. Nut! Ins& Metb. 116, 487,1974. First theoretical estimates of background in PIXE.

368


Chapter 6

E Folkmann. In: Ion Beam SufikekyerAndysi.(O.Meyer, G. Linker, and 0.Keppeler, eds.) Plenum, New York, 1976, pgs. 695 and 747. Early experimental evaluation of X-ray background in PIXE. 9 R G. Musket.In: Advances in X-RayAna&s. (G.J. McCarthy et al., eds.) Plenum Publishing, 1979, volume 22, p.401. PIXE of C on and in Fe. i o R. G. Musket. Nucl. ImtzMetb. Pbys.Res. B24/25,698, 1987. PIXE of 0 on and in Be. 11 J. L. Campbell and J. A. Cookson. Nucl. Imtz Metb. Pbys. Res. B3, 185, 1984. A review of thick-target PIXE. 12 R G. Musket. Nucl. Ins& Metb. Pbys. Res. 218,420, 1983. Examples of simultaneous P I E and RBS. 13 X. Zeng andX. Li. Nucl. Imtz Metb. Pbys.Res. B22,99, 1987. Details of k t , transistorized on-demand beam for PIXE. 14 J. L. Campbell, W. Maenhaut, E. Bombelka, E. Clayton, K. Malmqvist,J. A. Maxwell, J. Pallon, and J. Vandenhaute. Nucl. Instr.Metb. Pbys. Res. B14,204,1986. Comparison of five PIXE spectral processing techniques. 15 J. A. Cookson and J. L. Campbell. NucL Instr.Metb. Pbys.Res. 216,489, 1983. Calculated effects of surface roughness on thick-target PIXE. 8

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7 VISIBLE / U V EMISSION, REFLECTION, AND ABSORPTION Photoluminescence, PL 373 Modulation Spectroscopy 385 7.3 Variable Angle SpectroscopicEllipsometry, VASE

7.1 7.2

7.0

401

INTRODUCTION

In this chapter, three techniques using visible (and UV) light to probe the near surface regions of solids are described. In two of them, Photoluminescence, PL, and Modulation Spectroscopy, electronic transitions between valence and conduction bands excited by the incident light are used. Modulation Spectroscopy is simply a specialized way of recording the absorption spectrum as the wavelength of the incident light is scanned. The derivative spectrum is recorded by phase-sensitive detection as the temperature, electric field, or stress of the sample is modulated, improving sensitivity to small spectral changes. PLYon the other hand, looks at the emission spectrum as the excited electronic states induced by the incident light decay back to lower states. The third technique, Variable Angle SpectroscopicEllipsometry, VASE, involves measuring changes in intensity ofpokzrizedlight reflected from interfaces as a function of incident wavelength and angle. All three techniques probe 500 to 1 pm or so in depth for opaque materials, depending on the penetration depth of the incident light. For transparent materials, essentially bulk properties are measured by PL and Modulation Spectroscopy. All three techniques can be performed in ambient atmosphere, since visible light is used both as incident probe and signal. In Modulation Spectroscopy, which is mostly used to characterize semiconductor materials, the peak positions, intensities and widths of features in the absorption spectrum are monitored. The positions, particularly the band edge (which defines the band gap), are the most usefd, allowing determination of alloy concentration, 371

strain, and damage, and identificationof impurities. Absolute sensitivities are good enough to detect monolayer concentrations. Lateral resolution down to 100 pm can be achieved with suitable optics. Modulation Specnoscopy can sometimes be used as a screening method for device performance when there are h t u r e s in the spectrum (positions, widths, or intensities) that correlate with perhrmance. It can also be used as an in situ monitor of structural perfection while growing epitaxial

material. PL is currently more widely used than Modulation Spectroscopyin semiconductor and insulator characterization though it basically accesses some of the same information. It is also widely used outside the realm of semiconductor materials science, for example in pharmaceuticals, biochemistry, and medicine, where it is known under the general name of fluorometry. In PL of semiconductors, one monitors emission from the bottom of the conduction band to the top of the valence band. All the properties that can be determined from band-edge movement in Modulation Spectroscopy can also be determined by PL-for example, strain, damage, alloy composition, and the perfection of growing.&s Impurities or dopants can be detected with high sensitivitywhen there are transitions to impurity levels within the band gap. To sharpen transitions, and therefore improve resolution and sensitivity, PL is usually performed at cryogenic (liquid helium) temperatures. Under these conditions PL can detect species down to 10'0-10'4 cm3, depending on the particular species. Spatial resolution down to 1 pm can be obtained for fixed-wavelengthlaser incident PL. Instrumentation for PL and Modulation Spectroscopy is quite similar, often being constructed so that both can be performed. There is no complete ucommercial system." One usually builds up a system using commercial components (light source, monochromator, detector, etc.). Costs vary from a minimum of $10,000 for primitive detection of PL to $250,000 for a system that can do everything. PL has the same physical basii as Cathodoluminescence (CL) discussed in Chapter 3. The practical differences caused by the use of an electron beam are: probing depths can be shorter (down to 100 A); spatial resolution is better (dawn to 1000A); beam damagemay OCCUT, and a high-vacuum system is necessary. VASE monitors the intensity of polarized light afier it has been transmitted through a thin film to an interface, reflected, and transmitted back through the film. The film thickness (in the 1 nm to 1 pm range), the optical constants, and the interface roughness (if less than 100 nm) can all .be extracted if enough measurements at different angles of incidence are made. However, these parameters are derived from a fit to an assumed model (thickness and optical constants), which is not a unique procedure, leaving room for gross error in the wrong hands. Planar interfaces are needed over the lateral area probed, which is usually 1 mm2 but which can be as small as 100pm2. VASE is used primarily fbr surface coatings on semiconductors and dielectrics, optical matings, and mulalayer Kin-tilm structures. 372

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7.1

Photoluminescence CARL COLVARD

Contents

Introduction Basic Principles Common Modes of Analysis and Examples Sample Requirements Quantitative Abilities Instrumentation Conclusions

Introduction

Luminescence refers to the emission of light by a material through any process other than blackbody radiation. The term Photoluminescence (PL) narrows this down to any emission of light that results from optical stimulation. Photoluminescence is apparent in everyday life, for example, in the brightness of white paper or shirts (often treated with fluorescent whiteners to make them literally glow) or in the light from the coating on a fluorescent lamp. The detection and analysis of this emission is widely used as an analytical tool due to its sensitivity, simplicity, and low cost. Sensitivity is one of the strengths of the PL technique, allowing very small quantities (nanograms) or low concentrations (parts-per-trillion) of material to be analyzed. Precise quantitative concentration determinations are difficult unless conditions can be carefully controlled, and many applications of PL are primarily qualitative. PL is often referred to as fluorescence spectrometry or fluorometry, especially when applied to molecular systems. Uses for PL are found in many fields, including 7.1

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environmental research, pharmaceutical and food analysis, forensics, pesticide studies, medicine, biochemistry, and semiconductors and materials research. PL can be used as a tool for quantification, particularly for organic materials, wherein the compound of interest can be dissolved in an appropriate solvent and examined either as a liquid in a cuvette or deposited onto a solid surface like silica gel, alumina, or filter paper. Qualitative analysis of emission spectra is used to detect the presence of trace contaminants or to monitor the progress of reactions. Molecular applications include thin-layer chromatography (TLC) spot analysis, the detection of aromatic compounds, and studies of protein structure and membranes. Polymers are studied with regard to intramolecular energy transfer processes, conformation, configuration, stabilization, and radiation damage. Many inorganic solids lend themselves to study by PLYto probe their intrinsic properties and to look at impurities and defects. Such materials include alkalihalides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. Emission spectra of impurity levels are used to monitor dopants in 111-V, 11-VI, and group IV compounds, as well as in dilute magnetic and other chalcogenide semiconductors. PL efficiency can be used to provide a measure of surfice damage due to sputtering, polishing, or ion bombardment, and it is strongly affected by structural imperfections arising during the growth of films like S i c and diamond. Coupled with models of crystalline band structure, PL is a powerful tool for monitoring the dimensions and other properties of semiconductor superlattices and quantum wells (man-made layered structures with angstrom-scale dimensions). The ability to work with low light levels makes it well suited to measurements on thin epitaxial layers.

Basic Principles

In PLYa material gains energy by absorbing light at some wavelength by promoting an electron from a low to a higher energy level. This may be described as making a transition from the ground state to an excited state of an atom or molecule, or from the valence band to the conduction band of a semiconductor crystal (electron-hole pair creation). The system then undergoes a nonradiative internal relaxation involving interaction with crystalline or molecular vibrational and rotational modes, and the excited electron moves to a more stable excited level, such as the bottom of the conduction band or the lowest vibrational molecular state. (See Figure 1.) If the cross-coupling is strong enough this may include a transition to a lower electronic level, such as an excited triplet state, a lower energy indirect conduction band, or a localized impurity level. A common occurrence in insulators and semiconductors is the formation of a bound state between an electron and a hole (called 374

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stater Emitted

Emitted

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Crystalline Systems

Figure 1

Ground State

Molecular Systems

Schematic of PL from the standpoint of semiconductor or crystalline systems (left)and molecular systems (right).

an exciton) or involving a defect or impurity (electron bound to acceptor, exciton bound to vacancy, etc.) . After a system-dependent characteristic lifetime in the excited state, which may last from picoseconds to many seconds, the electronic system will return to the ground state. In luminescent materials some or all of the energy released during this final transition is in the form of light, in which case the relaxation is said to be radiative. The wavelength of this emission is longer than that of the incident light. This emitted light is detected as photoluminescence, and the spectral dependence of its intensity is analyzed to provide information about the properties of the material. The time dependence of the emission can also be measured to provide information about energy level coupling and lifetimes. In molecular systems, we use different terminology to distinguish between certain PL processes that tend to be fast (submicrosecond), whose emission we call fluorescence, and other, slower ones (lo4 s to 10 s) which are said to generate phosphorescence. The light involved in PL excitation and emission usually U s in the range 0.66 eV (roughly 200-2000 nm). Many electronic transitions of interest lie in this range, and efficient sources and detectors for these wavelengths are available. To probe higher energy transitions, UPS, X P S , and Auger techniques become useful. X-ray fluorescence is technically a high-energy form of PL involvingX rays and core electrons instead of visible photons and valence electrons. Although lower energy intraband, vibrational, and molecular rotational processes may participate in PL, they are studied more effectivelyby Raman scattering and IR absorption. Since the excited electronic distribution approaches thermal equilibrium with the lattice before recombining, only features within an energy range of -kT of the lowest excited level (the band edge in semiconductors) are seen in a typical PL emission spectrum. It is possible, however, to monitor the intensity of the PL as a hnction of the wavelength of the incidentlight. In this way the emission is used as a probe of the absorption, showing additional energy levels above the band gap. Examples are given below. 7.1

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band-edge Caccvtor mxcitons e-A defact nxcitons

T=ZK

phonon sideband

1.46

1.48

hew

Figure 2

1.50

1.52

(4

PL specba of MBE grown GaAs at 2 K near the fundamental gap, showing Cacceptor peak on a semilog scale.

Scanninga range of wavelengthsgives an emission spectrum that is characterized by the intensity, line shape, line width, number, and energy of the spectral peaks. Depending on the desired information, several spectra may be taken as a function of some external perturbation on the sample, such as temperature, pressure, or doping variation, magnetic or electric field, or polarization and direction of the incident or emitted light relative to the crystal axes. The features of the spectrum are then converted into sample parameters using an appropriate model of the PL process. A sampling of some of the information derived from spectral features is given in Table 1. A wide variety of different mechanisms may participate in the PL process and influence the interpretation of a spectrum. At room temperature, PL emission is thermally broadened. As the temperature is lowered, features tend to become sharper, and PL is often stronger due to fewer nonradiative channels. Low temperatures are typically used to study phosphorescence in organic materials or to identify particular impurities in semiconductors. Figure 2 shows spectra fiom high-purity epitaxial GaAs (NA < l O I 4 ~ m - at ~) liquid helium temperature. The higher energy part of the spectrum is dominated by electron-hole bound pairs. Just below 1.5 eV one sees the transition from the conduction band to an acceptor impuriry (+A). The impurity is identified as carbon from its appearance at an energy below the band gap equal to the carbon binding energy. A related transition from the acceptor to an unidentified donor state (=A) and a sideband lower in energy by one LO-phonon are also visible. Electrons bound to sites with deeper levels, such as oxygen in GaAs, tend to recombine nonradiatively and are not easily seen in PL. PL is generally most usell in semiconductors if their band gap is direct, i.e., if the extrema of the conduction and valence bands have the same crystal momentum, and optical transitions are momentum-allowed. Especially at low temperatures, 376


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Peak energy

Sample parameter

Compound identification Band gap/electronic levels Impurity or exciton binding energy Quantum well width Impurity species

May composition Internal strain Fermi energy

Peak width

Structural and chemical "quality" Quantum well interface roughness Carrier or doping density

Slope of high-energy tail

Electron temperature

Polarization

Rotational relaxation times Viscosity

Peak intensity

Relative quantity Molecular weight Polymer conformation Radiative efficiency Surfke damage Excited state lifetime Impurity or defect concentration

Table 1

Examples of sample parametersextracted from PL spectral data. Many rely on a model of the electronic levels of the particular system or comparison to standards.

localized bound states and phonon assistance allow certain PL transitions to appear even in materials with an indirect band gap, where luminescence would normally nor be expected. For this reason bound exciton PL can be used to identify shallow donors and acceptors in indirect GaP, as well as direct materials such as GaAs and 7.1

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InP, in the range 10'3-10'4 Boron, phosphorus, and other shallow impurities can be detected in silicon in concentrations' approaching 10'' ~ m - Copper ~. contamination at Si surfaces has been detected down to 10'' cm-3 levels.2 Common Modes of Analysis and Examples

Applications of PL are quite varied. They indude compositional analysis, trace impurity detection, spatial mapping, structural determination (crystallinity, bonding, layering), and the study of energy-transfer mechanisms. The examples given below emphasize semiconductor and insulator applications, in part because these areas have received the most attention with respect to surface-related properties (i.e., thin films, roughness, surface treatment, interfaces), as opposed to primarily bulk properties. The examples are grouped to illustrate four different modes for collecting and analyzing PL data: spectral emission analysis, excitation spectroscopy, time-resolved analysis, and spatial mapping. Spectral Emission Analysis

The most common configuration for PL studies is to excite the luminescence with fEed-wavelength light and to measure the intensity of the PL emission at a single wavelength or over a range of wavelengths. The emission characteristics, either spectral features or intensity changes, are then analyzed to provide sample information as described above. As an example, PL can be used to precisely measure the alloy composition x of a number of direct-gap 111-V semiconductor compounds such as AlxGal-&, InxGal-&, and GaAsxP1, since the band gap is directly related to x. This is possible in extremely thin layers that would be difficult to measure by other techniques. A calibration curve of composition versus band gap is used for quantification. Cooling the sample to cryogenic temperatures can narrow the peaks and enhance the precision. A precision of 1 meV in bandgap peak position corresponds to a value of 0.001 for x in AlxGal-&, which may be useful for comparative purposes even if it exceeds the accuracy of the x-versus-bandgap calibration. High-purity compounds may be studied at liquid He temperatures to assess the sample's quality, as in Figure 2. Trace impurities give rise to spectral peaks, which can sometimes be identified by their binding energies. The application of a magnetic field for magnetophotoluminescencecan aid this identification by introducing extra field-dependent transitions that are characteristic of the specific i m p ~ r i t yExamples .~ of identifiable impurities in GaAs, down to around 1013cm3, are C, Si, Be, Mn, and Zn. Transition-metal impurities give rise to discrete energy transitions within the band gap. Peak shifts and splitting of the acceptor-bound exciton lines can be used to measure strain. In heavily Be-doped GaAs and some quantum two-dimensional (2D) structures, the Fermi edge is apparent in the spectra, and its position can be converted into carrier concentration. 378


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1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90

Energy

Figure 3

(4

Composite plot of 2 K excitonic spectra from 11 GaAs/AI,~,Gap.,As quantum wells with different thicknesses. The well width of each is given next to its emission peak.

A common use of PL peak energies is to monitor the width of quantum well structures. Figure 3 shows a composite plot of GaAs quantum wells surrounded by AlO.3Ga0.7As barriers, with well widths varying from 13 nm to 0.5 nm, the last being only two atomic layers thick. Each of these extremely thin layers gives rise to a narrow PL peak at an energy that depends on its thickness. The well widths can be measured using the peak energy and a simple theoretical model. The peak energy is seen to be very sensitive to well width, and the peak width can give an indication of interface sharpness. PL can be used as a sensitive probe of oxidative photodegradation in polymers.' After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. PL Excitation Spectmscopy

Instead of scanning the emission wavelength, the analyzing monochromator can be fmed and the wavelength of the incident exciting light scanned to give a PL excitation (PLE) spectrum. A tunable dye or Ti:Sapphire laser is typically used for solids, or if the signals are strong a xenon or quartz-halogen lamp in conjunction with a source monochromator is sufficient. The resulting PL intensity depends on the absorption of the incident light and the mechanism of coupling between the initial excited states and the relaxed excited states that take part in emission. The spectrum is similar to an absorption spectrum and is usefid because it includes higher excited levels that normally do not appear in the thermalized PL emission spectra. Some transitions are apparent in PLE spectra from thin layers that would only be seen in absorption data if the sample thickness were orders of magnitude greater. This technique assists in the idenrification of compounds by distinguishing between substances that have the same emission energy but different absorption 7.1

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bands. In semiconductors, it can be valuable for identifylng impurity PL peaks, especially donors, by enhancing certain PL transitions through resonant excitation. It is useful for determining the energy levels of thin-film quantum structures, which, when combined with appropriate models, are used to simultaneouslydetermine well widths, interface band ofliets, and effective masses. Information about higher energy transitions can also be obtained by Modulation Spectroscopy techniques such as photoreflectance and electroreflectance. Time-ResolvedPL

By monitoring the PL intensity at a chosen wavelength as a function of time delay afier an exciting pulse, information can be obtained about the electron relaxation and recombination mechanisms, including nonradiative channels. The time scales involved may vary from two hundred kmtoseconds to tens of seconds. A 111 emission spectrum may be measured also at successive points in time. Spectral analysis then yields, for example, the evolution of a carrier distribution as excitonic srates form and as carriers are trapped by impurities. The progress of chemical reactions with time can be followed using time-dependent data. By monitoring the depolarization of luminescence with time of PL from polymer chains, rotational relaxation rates and segmental motion can be measured. A useful application of time-dependent PL is the assessment of the quality of thin 111-V semiconductor alloy layers and interfaces, such as those used in the fabrication of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the active region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at the cladding interface can rapidly deplete the free-carrier population, resulting in much shorter decay times. Measurements of lifetime are significantly less dependent on external conditions than is the PL intensity. PL Mapping

Spatial information about a system can be obtained by analyzing the spatial distribution of PL intensity. Fluorescent tracers may be used to image chemical uptake in biological systems. Luminescence profiles have proven useful in the semiconductor industry for mapping impurity distributions, dislocations, or structural homogeneity in substrate wafers or epilayers. Similar spatial information over small regions is obtained by cathodoluminescenceimaging. For mapping, the sample (or the optical path) is translated, and at each position PL at a single wavelength or over an entire spectrum is measured. The image is formed from variations in intensity, peak energy, or peak line width. Lateral resolution of 1 pm is possible. Figure 4 shows an application of PL to identify imperfections in a 2-in InGaAsP epitaxial wafer. 380


Chapter 7

Figure 4

Spatial variation of PL intensity of an InGaAsP epitaxial layer on a 2-in InP substrate shows results of nonoptimal growth conditions. (Data from a Waterloo Scientific SPM-200 PL mapper, courtesy of Bell Northern Research)

Sample Requirements

PL measurements are generally nondestructive, and can be obtained in just about any configuration that allows some optically transparent access within several centimeters of the sample. This makes it adaptable as an in situ measurement tool. Little sample preparation is necessary other than to eliminate any contamination that may contribute its own luminescence. The sample may be in air, vacuum, or in any transparent, nonfluorescing medium. Small probed regions down to 1-2 pm are possible using microscope lenses. Lasers can supply as much pump power as needed to compensate for weaker signals, but a limit is reached when sample heating or nonlinear optically induced processes become significant. For semiconductor work, either whole wafers or small pieces are used, the latter often being necessary for insertion into a cryostat. Bulk solids may be analyzed in any form, but scattered light may be reduced and the signal increased if the emitting surface is specular. Quantitative Abilities

Photoluminescence finds its greatest strengths as a qualitative and semiquantitative probe. Quantification based on absolute or relative intensities is difficult, although it is useful in applications where the sample and optical configurations may be carefully controlled. The necessary conditions are most easily met for analytical applica7.1

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tions of molecular fluorescence, where samples may be reproducibly prepared in the form of controlled filmsor as dilute concentrationsof material in a transparent liquid solvent, and where &rence standards are a~ailable.~ PL intensities are strongly influenced by h o r s like su& conditions, heating,

photochemical reactions, oxygen incorporation, and intensity, power density and the wavelength of the exciting light. If these fictors are carefdly controlled PL intensities can be used to studyvarious aspects of the sample, but such control is not always possible. Other aspects that can cause intensity variations are the focal region of the incident and collection optics, the relationship of the sample's image to the monochromator entrance slit, and the spectral response of the detector and optical path. Nevertheless, quantification is possible, a good example being the evaluation of the composition of chromatographic separations adsorbed onto glass, alumina, polyethylene, or paper. When compared with known standards, the presence of only a fkv nanograms of a strong fluorophore may be quanrified to better than 10%. As another example, PL from GaP:N at 77 K is a convenient way to assess nitrogen concentrations in the range 10'7-10'9 cm-3 by observing the ratio of the peak intensity of the nitrogen-bound exciton transition to that of its LO phonon sideband, or to peaks involving nitrogen pairs. Similar ratio analysis allows estimates of EL2 defect concentration in GaAs wafers and has been used to quanti+ Mn concentrations in GaAs. Under carefdly controlled conditions, PL intensity from layered-as-grown device structures can be correlated with device parameters (e.g., lasing threshold and transistor gain) and used to predict final device performance on other similar wafers. Instrumentation

A variety of commercial instruments are available for PL measurements. These include spectrofluorometers intended primarily for use with liquids in a standard configuration, and simple filter-based systems for monitoring PL at a single wavelength. For use with opaque samples and surfices, a few complete commercial systems are available or may be appropriately modified with special attachments, but due to the wide range of possible configuration requirements it is common to assemble a custom system fiom commercial optical components. Four basic components make up a PL system: 1 A source of light for excitation. Sur& studies generally require a continuous or pulsed laser. A dye or li:sapphire laser is used if tunability is needed. z A sample holder, including optics for hcusing the incident light and collecting the Iuminescence. Efficient light collection is important, and the sample holder may need to allow for a cryosmt, pressure cell, magnet, or electrical contacts.

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Chapter 7

Figure 5

Schematic layout of a high-sensitivity PL system incorporating a laser and photon-countingelectronics.

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. An optical detector with appropriate electronics and readout. Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range. Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients.

A schematic of a PL system layout is shown in Figure 5. This optical system is very similar to that required for absorption, reflectance, modulated reflectance, and Raman scattering measurements. Many custom systems are designed to perform several of these techniques, simultaneously or with only small modifications. Conclusions

Photoluminescence is a well-established and widely practiced tool for materials analysis. In the context of surface and microanalysis, PL is applied mostly qualitatively or semiquantitatively to exploit the correlation between the structure and composition of a material system and its electronic states and their lifetimes, and to identify the presence and type of trace chemicals, impurities, and defects. Improvements in technology will shape developments in PL in the near future. PL will be essential for demonstrating the achievement of new low-dimensional quantum microstructures. Data collection will become easier and Edster with the continuing development of advanced focusing holographic gratings, array and imaging detectors, sensitive near infrared detectors, and tunable laser sources. 7.1

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CL, Modulation Spectroscopy,Raman Spectroscopy,and FTIR References 1

I? J. Dean. Prog. CrystaLGrowth Charat. 5,89,1982. A review of PL as a diagnostic probe of impurities and defects in semiconductorsby an important progenitor of the technique.

2

L. T. Canham, M. R Dyball, and K. G. Barrad0ugh.J. Appl. Pbys. 66, 920,1989. G. E. Stillman, B. Lee, M. H. Kim, and S. S. Bose. h e . Elcchochem. Soc. 88-20,56, 1988. Describes the use of PL for quantitative impurity analysis in semiconductors. K D. Mielenz, ed. Measurement ofPhotoluminescence.vol. 3 of Optical Radiation Measuremena. (F. Grum and C. J. Bartleson, eds.) Academic Press, London, 1982. A thorough treatment of photoluminescence spectrometry for quantitative chemical analysis, oriented toward compounds in solution.

3

4

5

R J. Hurtubise. Solid Su$ae Luminescence Analysis. Marcel Dekker, New York, 1981. Practical aspects of analysis for organics adsorbed onto solids.

H. B. Bebb and E. W. Williams. in Semiconductorsand Semimetah. (R K. Willardson and A .C. Beers, e&.) Academic Press,vol. 8,1972. An extensive review of PL theory and technique, with emphasison semiconductors. Some of the experimental aspects and examples are becoming outdated. 7

H. J. Queisser. Appl. Pbys. 10,275, 1976. Describes PL measurements of a variety of semiconductor properties.

8

K Mettler. Appl. Pbys. 12,751977. PL measurements of surhce state densities and band bending in GaAs.

9

L. Zlatkevich, ed. Luminescence Techniquesin Solid-state Polymer Research. Marcel Dekker, New York, 1989. Practical emphasis on polymers in the solid state rather than in solution.

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7.2

Modulation Spectroscopy F R E D H. POLLAK

Contents

Introduction Basic Principles Instrumentation Line Shape Considerations Applications and Examples Conclusions

Introduction

Modulation Spectroscopy is an analog method for taking the derivative of an optical spectrum (reflectance or transmittance) of a material by modifying the measurement conditions in some manner.14 This procedure results in a series of sharp, derivative-like spectral features in the photon energy region corresponding to electronic transitions between the filled and empty quantum levels of the atoms that constitute the bulk or surfice of the material. Using Modulation Spectroscopy it is possible to meas-ure the photon energies of the interband transitions to a high degree of accuracy and precision. In semiconductors these band gap energies are typically 1 eV, and they can be determined to within a few meV, even at room temperature. The energies and line widths of the electronic transitions are characteristic of a particular material or surfice. The energies are sensitive to a variety of internal and external parameters, such as chemical composition, temperature, strains, and electric and magnetic fields. The line widths are a function of the quality of the material, i.e., degree of crystallinity or dopant concentration. The ability to measure the energy of electronic transitions and their line widths accurately, in a convenient manner, is one of the most important aspects of serniconductor characterization. The former can be used to evaluate alloy compositions 7.2

Modulation Spectroscopy

385

(including topographical scans)? near-surface temperatures? process- or growthinduced strainsY8 surface or intehce electric fields associated with surface or interhce states and metallization (Schottky barrier formation),8 carrier types,'" topographical variations in carrier concentrations? and trap states.8 The broadening parameter at a given temperature is a measure of crystal quality and hence can be used to evaluate the influence of various growth, processing and annealing procedures. These indude ion implantation, reactive-ion etching, sputtering, and laser or rapid annealing7s8In real device structures, such as heterojunction bipolar transistors, certain features of the Modulation Spectroscopy spectra have been correlated with actual device performance.6 Thus, this method can be employed as an effective screening tool to select materials having the proper device characteristics before undertaking an expensive fibrication process. Various forms of Modulation Spectroscopy can be employed for in-situ monitoring of growth by molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), or gas-phase MBE (GPMBE) at elevated temperatures." 3-"Modulation Spectroscopyhas been used extensively to study semiconductorshaving diamond (Ge and Si), zincblende (GaAs, GaAlAs, InP, CdTe, and HgCdTe), and m i t e (CdS) crystal structures. There also has been some work in the area of metals, including alloys. The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremelysharp. As a result, their wavelengths (photon energies) can be determined with great accuracy. The lines are characteristic of a particular atom or ion and can be used for identification purposes. Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp. Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules. The high particle density of solids, however, makes their optical spectra rather broad, and often uninteresting from an experimental point of view. The large degeneracy of the atomic levels is split by interatomicinteractions into quasicontinuous bands (valence and conduction bands). The energy difference between the highest lying valence and lowest lying conduction bands is designated as the fundam e n d band gap. Penetration depths for electromagnetic radiation are on the order of 500 A through most of the optical spectrum. Such small penetration depths (except in the immediate vicinity of the hndamental gap), plus other considerations to be discussed later, make the reflection mode more convenient for characterization purposes, relative to absorption measurements. These aspects of the optical spectra of solids are illustrated in the upper portion of Figure 1,which displays the reflectance curve (R) at room temperature for a typical semiconductor, GaAs.The hndamental absorption edge around 1.4 eV produces only a weak shoulder. Some structure is apparent in the two features around 3 eV and the large, broad peak near 5 eV. However, the dominant aspect of the line shape is the slowly varying background. The derivative nature of Modulation Spectroscopy suppresses the uninteresting background effects in hvor of sharp, deriva386

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I

I

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ENERGY (eV) Figure 1

Reflectance(R)and electroreflectance( A R I R )spectra of GaAs at 300 K.

tive-like lines corresponding to the shoulders and peaks in Figure 1. Also, weak structures that may go unseen in absolute spectra are enhanced. Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most useful techniques for studying the optical proponents of the bulk (semiconductorsor metals) and surface (semiconductors)of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors, Modulation Spectroscopy is gaining in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 7.2


387

Basic Principles

The basic idea of Modulation Spectroscopy is a very general principle of experimental physics. Instead of measuring the optical reflectance (or transmittance) of a material, the derivative with respect to some parameter is evaluated. The spectral response of the material can be modified directly by applying a repetitive perturbation, such as an electric field (electromodulation), a heat pulse (thermomodulation), or stress (piezomodulation).This procedure is termed external modulation. The change may also occur in the measuring system itself, e.%., the wavelength or polarization conditions can be modulated or the sample reflectance (transmittance) can be compared to a reference sample. This mode has been labeled intemalmodulation. Because the changes in the optical spectra are typically small, in some cases 1 part in lo6, phase-sensitive detection or some other signal-processing procedure is required. To illustrate the power of Modulation Spectroscopy, displayed in the lower part of Figure 1 is the electromodulated reflectance spectra ( A R / R ) of the semiconductor GaAs at 300 K in the range 0-6 eV. Although the fundamental direct absorption edge (E,)at about 1.4 eV produces only a weak shoulder in R it is observed as a sharp, well-resolved line in AR/ R There are also other spectral features, labeled 4 + Ao, El, E1 + AI, &, and E2 that correspond to transitions between other quantum levels in the semiconductor. In the region of the features at E, and E, + A0 the penetration depth of the light (the sampling depth) is typically several thousand A, while for the peaks at El and E1 + 8 1 the light samples a depth of only a few hundredA. For characterizationpurposes of bulk or thin-film semiconductorsthe features at E, and El are the most useful. In a number of technologically important semiconductors (e.g., Hgl+Cd,Te, and In,Gal-&) the value of E, is so small that it is not in a convenient spectral range for Modulation Spectroscopy, due to the limitations of light sources and detectors. In such cases the peak at E1 can be used! The features at & and 4 are not useful since they occur too far into the near-ultraviolet and are too broad.

Instrumentation Gttemal Modulation

For characterization purposes the most useful form of external modulation is electromodulation, because it provides the sharpest structure (third derivative of R in bulk or thin films) and is sensitive to surfice or interhce electric fields.'-5 The most widely used contactless mode of electromodulation is termed Photoreflectance

(PR).53 3

388

'


Chapter 7

LAMP

LASER (OR QTHER SECONDARY LIGHT SOURCE)

Figure 2

Schematic representationof a photoreflectanceapparatus.

A schematic representation of a PR apparatus is shown in Figure 2.’ In PR a

pump beam (laser or other light source) chopped at frequency a, creates photoinjected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerhl pump must be employed.) Light from an appropriate light source (a xenon arc or a halogen or tungsten lamp) passes through a monochromator (probe monochromator). The exit intensity at wavelength A, I&), is focused onto the sample by means of a lens (or mirror). The reflected light is collected by a second lens (mirror) and hcused onto an appropriate detector (photomultiplier, photodiode, etc.). For simplicity, the two lenses (mirrors) are not shown in Figure 2. For modulated transmission the detector is placed behind the sample. The light striking the detector contains two signals: the dc (or average value) is given by I&)R(A), where R(A)is the dc reflectance of the material, while the modulated value (at frequency Q), is IO(h)AR(h),where AR(h) is the change in reflectance produced by the modulation source. The ac signal from the detector, which is 7.2


389

proportional to IOAR, is measured by a lock-in amplifier (or using another signalaveraging procedure). Typically IoAR is 104-104 IoR To evaluate the quantity of interest, i.e., the relative change in reflectance, AR/R, a normalization procedure must be used to eliminate the uninteresting common feature I&). In Figure 2 the normalization is performed by the variable neutral density frlter (VNDF) connected to a servo mechanism. The dc signal from the detector, which is proportional to Io(h)R(h),is introduced into the servo, which moves the VNDF in such a manner as to keep I&)R(A) constant, i.e., I,(h)R(h)= C. Under these conditions the ac signal I&)AR(h) = C&R(A)/R(h). Commercial versions of PR are available. Other contactless methods of electromodulation are Electron-Beam Electro-reflectance (EBER)l2 and Contactless Electroreflectance (CER)13. In EBER the pump beam of Figure 2 is replaced by a modulated low-energy electron beam (- 200 ev) chopped at about 1 kHz. However, the sample and electron gun must be placed in an ultrahigh vacuum chamber. Contactless electroreflectance uses a capacitor-like arrangement. An example of a contact mode of electromodulation would be the semiconductor-insulator-med configuration, which consists of a semiconductor, about 200 of an insulator like AlZO3, and a semitransparent metal (about 50 A of Ni or Au). Modulating (ac) and bias (dc) voltages are applied between the front semitransparent metal and a contact on the back of the sample. T o employ this mode the sample must be conducting. In temperature modulation, the sample may be mounted on a small heater attached to a heat sink and the temperature varied cyclically by passing current pulses through the heater.' If the sample is properly conducting, the current can be must be kept passed through the sample directly. Generally, for this method below 10-20 Hz, and hence there are often problems with the l/f noise of the detector. In piezoreflectance (PzR), modulation is achieved by mounting the sample on a piezoelectric transducer that varies the lattice constant of the material, producing a band gap modulation. l4 Although PzR is contactless it requires special mounting of the sample, as does thermomodulation.

a

a,

Internal Modulation

Differential Reflectivity

A commonly used form of internal modulation is differential reflectometry, in which the reflectance of the sample under investigation (or a portion of it) is compared to a standard material. This can be accomplished either by holding the sample stationary and scanning the probe beam between two region^'^ or by holding the light spot fixed and moving the sample."

390


Chapter 7

Reflection Difference Spectroscopy

In Reflection Difference Spectroscopy (RDS) the difference between the normalincidence reflectance R of light polarized parallel and perpendicular to a principal crystallographica x i s in the plane of the crystal is measured experimentally as a function of time, photon energy, or surfice conditi~ns.~-'lBecause of The cubic symmetry of zincblende semiconductors, the bulk is nearly isotropic (i.e., there is no distinction between parallel and perpendicular), while regions of lower symmetry, like the surface or interfaces can be anisotropic. In the case of (001) surfices of zincblende semiconductors, the contribution from the bulk is expected to vanish. Thus, RDS is sensitive to both the chemical and structural state of the surface. Sensitivities to surface species of 0.01 monolayer have been demonstrated, with averaging times of 100 ms. Being an optical probe, RDS is well suited either to the reactive, relatively high-pressure sample environments in MOCVD reactors or to the ultrahigh-vacuum environment of MBE chambers. Moreover, the presence of a film deposited on the viewport can be overcome. Line Shape Considerations

One of the great advantages of Modulation Spectroscopy is its ability to fit the line shapes of sharp, localized structures, as illustrated in the lower part of Figure 1. These fits yield important relevant parameters, such as the value of the energy gap and the broadening parameter. Electromodulation

The most complicated form of Modulation Spectroscopy is electromodulation, since in certain cases it can accelerate the electron-hole pairs created by the light. If the electric field is not too large the quantity AR/ R can be written as:

where A is the amplitude of the signal, @ is phase angle that mixes together the real and imaginary parts of dielectric function, E is the photon energy, Eg is the energy gap and r is a parameter that describes the broadening of the spectral line. The parameter m = 2.5 or 3.0 for the & , and E1 optical features, respectively. Equation (1) is related to the third derivative of R At low temperatures the electron and hole created by the probe light beam can form a bound state (called an exciton) because of the Coulomb interaction between them. In this case the exponent m in Equation (1) becomes 2 and the line shape is only a first For sufficiently high built-in electric fields the electromodulation spectrum can 7.2


391

display an oscillatory behavior above the band gap; these are called Franz-Keldysh oscillations (FK oscillations). In the presence of the field F the energy bands are tilted by an amount eFz, where e is the electronic charge and z is in the direction of F. Resonances appear whenever an integral number of de Broglie wavelengths fit into the triangular well formed by the electric field. The de Broglie wavelength is equal to 4n2/bp, where h is Planck’s constant and p is the momentum of the electron (hole). The energy of the mh resonance E,, is proportional to p .Thus the periods of these resonances, or FK oscillations, are a direct measure of the built-in electric Piezo- and Thermomodulation

These modulation methods do not accelerate the electron-hole pairs and hence produce only a first-derivative Modulation Spectroscopy. Their line shapes are given by Equation (l), with m = 2. Applications and Examples Alloy composition

Among the most important parameters for materials characterization are the compositions of binary A,, B, (e.g., Gel, Si,) alloys, ternary A,-, B, C (e.g., Gal, Al, As ,Hg l,Cdx Te 1 alloys, and quaternary A,, B, CyD1, (e.g., Inl-xGa&+’l-y) alloys. The spectral features in Figure 1, e.g., 4 and El vary with alloy composition. Modulation Spectroscopy thus can be employed conveniently for this purpose even at 300 K. Shown in Figure 3 is the variation of the fundamental direct band gap (4)of Gal-Jil& as a function of Al composition (x). These results were obtained at 300 K using electromodulation.Thus it would be possible to evaluate the Al composition of this alloy from the position of b. The case of Gal-fi& alloy determination is an example of the importance of the reflectance mode in relation to transmittance. In almost all cases the Gal,Al,Asmaterial is an epitaxial film (0.1-lpm) grown on a GaAs substrate (-0.5 mm thick). Since the band gap of GaAs is smaller than that of Gal-A&, the reflectance mode must be used. have a value of 4 in certain composition Some materials, such as Hgl-.$d,Te, regions that is too fir into the infrared to be conveniently observed using Modulation Spectroscopy. In such circumstances other higher lying features, such as the peaks at E1,can be used more readily. The compositional variation of 4 or higher lying features has been reported for a large number of alloys, including GeSi, GaAlAs, GaAlSb, G A P , InGaAs, InAsSb, InAsP, GaInSb, HgCdTe, HgMnTe, CdMnTe, CdZnTe, ZnMnTe, CdMnSe, InGaAsP lattice-matched to InP, GaAlInAs lattice-matched to InP, and 392


Chapter 7

0

w

0

0.2

0.4

0.6

0.8

1.0

Composition x Figure 3

Aluminum composition dependence of E, of Ga,,AI,+

at 300 K (solid line).

GaAlInPAs lattice-matched to GaAs. The alloy composition x can be evaluated with a precision of h = f 0.005. By using a high-quality lens to focus the light from the probe monochromator onto the sample (see Figure 2) a spot size of about 100 can be achieved. By mounting the sample on an x-y stage it is possible to perform topographical scans with a spatial resolution of 100 pm. Growth or Process-Induced Strain or Damage

Modulation Spectroscopy can be very useful in evaluating strains induced by growth (lattice-mismatched systems) or processing procedures, such as reactive-ion etching or oxide formation. The size and magnitude of the strain can be evaluated from the shifts and splittings of various spectral lines, such as 4 or El Device Structures Certain features in the

PR spectra at 300 K from GaAs/Gal-a&

heterojunction bipolar transistor structures have been correlated with actual device performance; thus PR can be used as an efkctive screening tool.6 From the observed FK oscillations it has been possible to d u a t e the built-in dc electric fields Fdc in the Gal-$& emitter, as well as in the n-GaAs collector region. The behavior of Fdc ( G U ) has been found to have a direct relation to a c t d device performance, i.e., dc current gain. Shown in Figures 4a and 4b are the PR spectrum at 300 K for MBE and MOCVD fabricated samples, respectively. There are a number of FK oscillations in the vicinity of both the GaAs (-1.42 eV) and Gal-$& 7.2


393

E E

\

a

1.5

I .8

2.I

2.4

Energy (eV) Figure 4

Photoreflectance spectra for two GaAs/Ga,-&As heterojunction bipolar transistor structures fabricated by MBE and MOCVD, respectively, at 300 K.

band gaps. The Gal-$& portions of the two samples are 1.830 eV and 1.670 eV, which corresponds to x = 0.28 and 0.17, respectively, as shown in Figure 3. The most important aspects of Figure 4 are the FK oscillations associated with the Gal-A& band gap. From these features it is possible to evaluate Fdcin the emitter-base p-n junction. The electric fields, as deduced from the GaAlAs FK osdllations (Fdc,GaAlAs), were compared with fabricated heterojunction bipolar transistor MBE samples. Below electric field values of about 2 x lo5 V/cm high current gains were obtained. Shown in Figure 5 is FdC(in GaAlAs) as a hnction of dc current gain at 1 mA. Note that there is a sudden drop when Fdc (in GaAlAs) > 2 x lo5 V/cm. The explanation of this effect is the redistribution of the Be dopant in the p-region in these MBE samples. When the redistribution moves the p n junction into the emitter, there is an increase in the electric field in this region; i.e., the value of Fdcbecomes greater. The movement of the Be has been verified by Secondary Ion Mass Spectroscopy (SIMS). When the p n junction and the GaAs /G& heterojunction are not coincident, carrier recombination occurs, reducing the current and the performance of fabricated heterojunction bipolar transistors. These observations have made it possible to use PR as a contactless screening technique to eliminate wafers with unwanted characteristics before the costly fabrication step. 394

...


Chapter 7

601

0

00

m

[I

40-c

20 0

0

O

n

OO 0 .

0 I

n ,

Fdc (GaAIAs) [IO5 V/cm] Figure 5

Electricfield ' F (GaAlAs) in the p n junction as evaluatedfrom the GaAlAs FK oscillations as a function of the dc current gain of a fabricated heterojunction bipolar transistor.

In-Situ Monitoring of Growth

RDS and PR are proving to be very useful methods for in-situ characterization of semiconductor thin-film growth by MBE, MOCVD, and GPMBE. RDS was first applied to study GaAs growth in an MBE environment. Results showed that the maximum surfice anisotropy between (2 x 4) As-terminated and (4 x 2) Ga- and AI-terminated surfaces of GaAs and ALAS occur in the photon energy region between 2.0-2.5 eV and 3.5 eV, respectively. The strong dependence of this anisotropy on photon energy makes it possible to spectrally distinguish between AI-AI and Ga-Ga surface dimer bonds. The time dependence of RDS and simultaneously measured reflection high-energy electron diffraction (RHEED) signals for changes in surface conditions revealed that the RDS measurements follow surface structure. The RDS-RHEED correlation gives a valuable reference when RDS is applied to nonultrahigh-vacuum techniques, such as MOCVD, where RHEED cannot be used. A commercial model of an RDS system is available. MBE Growth Studied by RDS

Figure 6 shows typical RDS (bottom) and RHEED (top) responses for an As-toGa-to-As surface stabilization sequenc-from As-stabilized (2 x 4) to Ga-stabilized (4 x 2) (001) surface reconstructions and return-generated by interrupting and resuming the As flux at times t = 1 s and 10 s, respectively, during otherwise normal growth of GaAs at a rate of 1 GaAs monolayer per 4.6 s . ~ ,lo The As growthsurface pressure of 6 x lo4 torr provided 2.6 times the amount needed to consume the arriving Ga. The differences between the RDS data on the left and those on the right are due to the differences in energy of the photons used to obtain them. The differences in the RHEED data are due to small angle-of-incidence drifts of the electron beam in the time interval between the recording of successive sets of data. 7.2


395

Figure 6

RHEED (upper)and reflection anisotropy (lower)transients obtained by interrupting and resumingAs flux during otherwise normal growth (001) GaAs at 1 semiconductor ML per 4.6 s.Data are shown for photon energies near the Ga RD peak at 2.5 eV (right)and minimum at 3.5 eV (left).

The maximum change of the 2.48-eV RDS signal is nearly 1%, with Rl increasing relative to RI 10 as the surface becomes increasingly covered with Ga. As soon as the As flux is terminated, the RDS signal begins to change nearly linearly in time, and saturates near t = 5 s; i.e., it tracks the amount of excess Ga accumulating on the surface up to one monolayer. Since RDS responds only to surface species that are in registry with the crystallographic axes of the substrate (i.e., have already reacted with it), and since it is insensitive to the presence of randomly oriented species, t h i s time dependence implies that the excess Ga atoms are forming Ga-Ga dimer bonds instantly on arrival, with respect to laboratory time scales, and that the 2.48-eV RDS signal directly follows the chemistry of the (001) GaAs growth surface. It also implies that Ga diffusion lengths under Ga-stabilized surface conditions are large, in particular, hundreds of times greater than under As-stabilized conditions. 396

...


Chapter 7

The 3.54-eV RDS response is completely different, exhibiting a striking similarity to the W E E D signal shown above it. Clearly, at this photon energy the RDS signal, as W E E D , is determined by surface structure. Thus RDS data either can complement or supplement W E E D data, depending on the measurement wavelength. As the saturation RDS signal at 3.54 eV is about an order of magnitude smaller than that at 2.48 eV, it follows that the small inflection in the otherwise linear initial 2.48-eV RD transient is due to the contribution of the structuresensitive component, which is relatively minor at the lower photon energy. Substrate Temperatureand Alloy Composition by PR

It has been demonstrated that PR can be used to measure E, of technologically important materials, such as GaAs, InP, Ga()@0.18h, and InxGal-& (x= 0.06 and 0.15), to over 600" C.6*7 Such temperatures correspond to growth conditions for thin-film methods like MBE, MOCVD, and gas-phase MBE. The value of 4 can be evaluated to f 5 meV at these elevated temperatures. Thus, the temperature of GaAs and InP substrates can be evaluated to f10" C to within a depth of only several thousand A from the growth surface. In addition, the alloy composition of epilayers of Gal+Al& and InxGal-& can be determined during actual growth. Measurements have been performed under actual growth conditions, including the case of a rotating substrate. Topographical scans can be performed to evaluate temperature or compositional homogeneity. Figure 7 shows 4 for GaAs and Ga0&&18h as a function of temperature T to about 900 K. Additional measurements on samples having differing AI contents would generate a family of curves. The solid line is a least-squares fit to a semiempirical relation that describes the temperature variation of semiconductor energy gaps:

aTL

E ( T ) = E ( 0 ) --

P+ T

In Equation (2) E(0) is the energy gap at T =0, while a and p are materials parameters to be evaluated from experiment. Once the GaAs substrate temperature is measured from the position of E,(GaAs), the AI composition of an epilayer can be determined readily from the position of 4 (GaAlkr) at that temperature. Conclusions

Modulation Spectroscopy has proven to be an important characterization method for semiconductors and semiconductor microstructures. The rich spectra contain a wealth of information about relevant materials, surfaces and interfaces, as well as device characteristics. In general, the apparatus is relatively simple, compact (except EBER), inexpensive (except EBER), and easy to use. One of the main advantages of Modulation Spectroscopy is its ability to perform relevant measurements at room 7.2


397

Temperature

Figure 7

(K)

Temperature dependence of of GaAs (circles) and Ga0.82AI0.18As(squares). The solid lines are least-squaresfm to Equation (2).

temperature (or even above). Several modulation techniques, such as PR and RDS, not only are contactless but also require no special mounting of the sample and can be performed in any transparent ambient. In bulk or thin films, material properties like the alloy composition (including topographical variations), the near-surfice temperature, growth- or processinduced strain or damage, the influence of annealing procedures, surfice or interfice electric fields associated with Fermi level pinning, carrier types, topographical variations in carrier concentrations, and trap states, can be determined. Various contactless modulation methods, such as PR and RDS,can produce valuable information about surface and interface phenomena, including crystal growth at elevated temperatures. In real device structures like heterojunction bipolar transistors, certain features in the PR spectrum can be correlated with actual device performance. Thus PR has been employed as an effective contactless screening technique to eliminate structures that have unwanted properties.

398

VISIBLE/UV EMISSION, REFLECTION, ...

Chapter 7

A major thrust in the hture will be the use of contactless modulation methods like PR or RDS (together with scanning ellipsometry) for the in-situ monitoring and control of growth and processing, including real-time measurements. These methods can be used not only during actual growth at elevated temperatures but also for in-situ post growth or processing at room temperature before the sample is removed from the chamber. Such procedures should improve a material’s quality and specifications, and also should serve to reduce the turn-around time for adjusting growth or processing parameters. The success of PR as a contactless screening tool for an industrial process, i.e., heterojunction bipolar transistor structures, certainly will lead to more work on real device configurations. There also will be improvements in instrumentation and software to decrease data acquisition time. Changes can be made to improve lateral spatial resolution. For example, if the probe monochromator is replaced by a tunable dye laser spatial resolutions down to about 10 pm can be achieved. Related Articles in the Encyclopedia

RHEED, VASE

References Semiconductorsand Semimetah. (R K. Willardson and A. C. Beer, eds.) Academic, New York, 1972, Volume 9. z Proceedings of the First International Conference on Modulation Spectroscopy. Su$ Sci. 37, 1973. 1

3

D. E. Aspnes. In: Handbook on Semiconductors.(T. S . MOSS, ed.) North Holland, New York, 1980, Volume 2, p. 109.

4

E H. Pollak. h c . SOC. Photo-OpticalImtz Eng. 276,142, 1981. E H. Pollak and 0.J. Glembocki. h c . SOC. Photo-OpticalImtz Eng. 946, 2, 1988. D. E. Aspnes, R Bhat, E. Coles, L. T Florez, J. I? Harbison, M. K. Kelley, V.G. Keramidas, M. A. Koza, and A. A. Studna. Proc. SOC. Photo-Optical Imtz Eng. 1037,2,1988. D. E. Aspnes, J. I? Harbison, A. A. Studna, and L. T. F1orez.J Vac. Sci. Tecbnol. A6, 1327, 1988. E H. Pollak and H. Shen. J Crystal Growth. 98,53,1989. R Tober, J. Pamulapari, R K. Bhattacharya, and J. E. Oh. J Ekmonic Mater. 18,379, 1989. B. Drevillon. Proc. SOC. Photo-OpticalInstz Eng. 1186,110, 1989.

5

6

7

8 9

10

7.2


399

11

E H. Pollak and H. Shen./. E&ctronicMat. 19,399,1990.

i z Proceedings of the International Conference on Modulation Spectroscopy. Proc. SOC. Photo-OpticalIns& Eng. 1286,1990. 13 14

M. H.Herman. h o c . SOC.Photo-OpticalInstr. Eng. 1286,39,1990. R E.Hummel, W. Xi, and D. R. Hagmann./. E&cmchm. SOC.137, 3583,1990.

400

VIS!BLE/UV EMISSION, REFLECTION,

...

Chapter 7

7.3

VASE Variable Angle Spectroscopic Ellipsometry J O H N A . WOOLLAM A N D P A U L G . S N Y D E R

Contents

Introduction Basic Principles Applications Conclusions

Introduction

The technique of ellipsometry was introduced in the 1 8 0 0 ~but ~ until computers became available, it was painllly slow to perform.' Rapid advances in small computer technology have made ellipsometric data acquisition rapid and accurate. Most impormnt, kt personal computers make possible quick and convenient analysis of data from complex material structures. Early work in ellipsometry focused on improving the technique, whereas attention now emphasizes applications to materials analysis. New uses continue to be found; however, ellipsometry traditionally has been used to determine frlm thicknesses (in the range 1-1000 nm), as well as optical constants.14 Common systems are oxide and nitride films on silicon wafers, dielectric films deposited on optical suhces, and multilayer semiconductor structures. In ellipsometry a collimated polarized light beam is directed at the material under study, and the polarization state of the reflected light is determined using a second polarizer. To maximize sensitivity and accuracy, the angle that the light makes to the sample normal (the angle of incidence) and the wavelength are controlled.u The geometry of a typical ellipsometry set up is shown in Figure 1. Ellipsometry is a very powerfd, simple, and totally nondestructivetechnique for determining optical constants, film thicknesses in multilayered systems, surface and 7.3

VASE

401

I

I

Figure 1

Planar structure anumedfor ellipsometric analysis: 4is the complex index of refraction for the ambient medium; n, is the complex index for the substrate medium; 0, is the value of the angles of incidence and reflection, which define the plane of incidence.

interfacial roughness, and material microstructures. (An electron microscope may alter surkces, as may Rutherford backscattering.) In contrast to a large class of surface techniques such as ESCA and AUGER, no vacuum chamber is necessary in ellipsometry. Measurements can be made in vacuum, air, or hostile environments like acids. The ability to study surfices at the interface with liquids is a distinct advantage for many disciplines, including surface chemistry, biology and medicine, and corrosion engineering. Ellipsometry can be sensitive to layers of matter only one atom thick. For example, oxidation of freshly cleaved single-crystal graphite can be monitored from the first monolayer and up. The best thicknesses for the ellipsometric study of thin films are between about 1 nm and 1000 nm. Although the spectra become complicated, films thicker than even 1 pm can be studied. Flat planar materials are optimum, but surface and interfacial roughness can be quantitatively determined if the roughness scale is smaller than about 100 nm. Thus ellipsometry is ideal for the investigation of interhcial surfaces in optical coatings and semiconductor structures?’ 7 In some applications lateral homogeneity of a sample over large areas needs to be determined, and systems with stepper driven sample positioners have been built. Use of focused ellipsometer beams is then highly desirable. As normally practiced, the lateral resolution of ellipsometry is on the order of millimeters. However, the light beam can be focused to 100 pn if the angle of incidence variation is not criti d . For smaller focusing the beam contains components having a range of angles of incidence that may alter the validity of the data analysis. Depth resolution depends on the (spectrallydependent) optical absorption coefficient of the material. Near-surface analysis (first 50 nm) frequently can be per43

-

402

VISIBLE/UV EMISSION, REFLECTION, ...

Chapter 7

b

x, y components

E.

Propagation direction

Figure 2

(a) Representation of a linearly polarized beam in its x- and p or (p and s-) orthogonal component vectors. The projection plane is perpendicularto the propagation direction; (b) lows of projection of electric vector of light wave on the projection plane for elliptically polarized light-a and b are the major and minor axes of the ellipse, respectively, and a is the azimuthal angle relative to the x-axis.

formed using short wavelength light (2300 nm) where absorption is strongest, and infiared radiation probes deeply (many pm) into many materials, including semiconductors. Basic Principles Light Waves and Polarization

Light is an electromagnetic wave with a wavelength ranging from 350 nm (blue) to 750 nm (red) for visible radiation.8 These waves have associated electric (E) and magnetic ( H ) components that are related mathematically to each other, and thus the Ecomponent is normally treated alone. Figure 2a shows the electric field associated with linearly polarized light as it propagates in space and time, separated into its x- and y-vector components. In the figure the x- and ycomponents are exactly in phase with each other thus the electric vector oscillates in one plane, and a projection onto a plane perpendicular to the beam propagation direction traces out a straight line, as shown in Figure 2a. When the vector components are nor in phase with each other, the projection of the tip of the electric vector onto a plane perpendicular to the beam propagation direction traces out an ellipse, as shown in Figure 2b. A complete description of the polarization state includes:' 1 The azimuthal angle of the electric field vector along the major axis of the ellipse (recall the angle a in Figure 2b) relative to a plane of reference 7.3

VASE

403

2

The ellipticity, which is defined by e = b / a

3

The handedness (righthanded rotation of the electric vector describes clockwise rotation when looking into the beam)

4

The amplitude, which is defined by A = (a2+ 62)45

5

The absolute phase of the vector components of the electric field.

In ellipsometry only quantities 1 and 2 (and sometimes 3) are determined. The absolute intensity or phase of the light doesn't need to be measured, which simplifies the instrumentation enormously. The handedness information is normally not critical. All electromagnetic phenomena are governed by Maxwell's equations, and one of the consequences is that certain mathematical relationships can be determined when light encounters boundaries between media. Three important conclusions that result for ellipsometry are: '3

1

The angle of incidence equals the angle of reflectance 80 (see Figure 1).

z Snell's Law holds: nl sin el = sin 0, (Snell's Law), where nl and are the complex indexes of refraction in media 1 and media 0, and the angles 8 , and00 are shown in Figure 1. 3

The Fresnel reflection coefficients are:

- nocosel ~ d -p nlmSeO nl coseo+ noCOSel

r =-P

Edp

where 5 refers to the light vector component perpendicular to the plane of incidence, p refers to the component parallel to the plane of incidence, and rand i refer to reflected and incoming light. The plane of incidence is defined by the incoming and outgoing beams and the normal to the sample. The complex indices of refraction for media 0 and 1 are given by and nl. The relations r, and rp are the complex Fresnel reflection coefficients. Their ratio is measured in ellipsometry: r P = ( t a n Y ) e iA p = -

5 Since p is a complex number, it may be expressed in terms of the amplitude factor tan Y, and the phase factor exp jA or, more commonly, in terms of just Y and A. Thus measurements of Y and A are related to the properties of matter via Fresnel coefficients derived from the boundary conditions of electromagnetic theory.

',

404


Chapter 7

There are several techniques for measuring Y and A, and a common one is discussed below. Equations l a and 1b are for a simple two-phase system such as the air-bulk solid intehce. Real materials aren't so simple. They have natural oxides and surface roughness, and consist of deposited or grown multilayered structures in many cases. In these cases each layer and interface can be represented by a 2 x 2 matrix (for isotropic materials), and the overall reflection properties can be calculated by matrix multiplication.' The resulting algebraic equations are coo complex to invert, and a major consequence is that regression analysis must be used to determine the system's physical parameters.'' 2, In a regression analysis Y t and A t are calculated from an assumed model for the structure using the Fresnel equations, where Y and A in Equation 2 are now indexed by c, to indicate that they are calculated, and by i, for each combination of wavelength and angle of incidence. The unknown parameters of the model, such as film thicknesses, optical constants, or constituent material fractions, are varied until a best fit between the measured Yi" and Aim and the calculated Y t and A i is found, where m signifies a quantity that is measured. A mathematical function called the mean squared error (MSE) is used as a measure of the goodness of the fit: 53

MSE =

N

-C (Y;-Y?) N 1

2

+(

2

A ~ ~ - A ~ ~ )

(3)

i= 1

where N is the number of wavelength and angle of incidence combinations used. The MSE is low if the user's guess at the physical model for the system was correct and if starting parameters were reasonably close to correct values. The model-dependent aspect of ellipsometric analysis makes it a difficult technique. Several different models fit to one set of data may produce equivalently low MSEs. The user must integrate and evaluate all available information about the sample to develop a physically realistic model. Another problem in applying ellipsometry is determining when the parameters of the model are mathematically correlated; for example, a thicker fdm but lower index of refraction might give the same MSE as some other combinations of index and thickness. That is, the answer is not always unique. Access to the correlation matrix generated during the regression analysis is thus important's to determine which, and to what degree, variables are correlated. It is common for the user of an ellipsometer-mistakenlyto make five wrong (correlated) measurements of an index of refraction and film thickness at, say, 632.8 nm and then to average these meaningless numbers. In reality all five measurements gave nonunique values, and averaging is not a valid procedure-the average of five bad numbers does not yield a correct number! The solution to the correlation problem 7.3

VASE

405

tr.f4

Toughness

ta,fa t4,fZ tl

Substrate

Figure 3

Common structure assumed for ellipsometric data analysis: tl and lj are the thicknessas of the two deposited films, for example; and t, are interfacial and surface roughness regions; 4 is the fraction of film tl mixed with film lj in an effective medium theory analysis of roughness-film f3 could have void (with fraction 1-41 dispersed throughout; and f, is the fraction of t, mixed with the ambient medium to simulate surface roughness.

is to make many measurements at optimum wavelength and angle combinations, and to keep the assumed model simple yet realistic. Even then, it is sometimes inherently not possible to avoid correlation. In this case especiallyit is important to know the degree. of correlation. Predictive modeling can be performed prior to making any measurements to determine the optimum wavelength and angle combinations to use, and to determine when there are likely to be correlated variables and thus nonunique an~wers.~’ A typical structure capable of being analyzed is shown in Figure 3, consisting of a substrate, two films (thicknesses tl and t3), two roughness regions (one is an interfacial region of thickness %, and the other is a surface region of thickness t4). One of the films t l or t3 may consist of microscopic (less than 100 nm size) mixtures of two materials, such as SiO, and Si3N4. The volume ratios of these two constituents can be determined by ellipsometry using effective medium theory. lo This theory solves the electromagneticequations for mixtures of constituent materials using simplifying approximations, resulting in the ability of the user to determine the fraction of any particular species in a mixed material. Likewise the roughness layers are modeled as mixtures of the neighboring media (air with medium 3 for the surface roughness, and medium 1 with medium 3 for interfacial roughness, as seen in Figure 3). The example in Figure 3 is as complex as is usually possible to analyze. There are seven unknowns, if no indices of refraction are being solved for in the regression analysis. If correlation is a problem, then a less complex model must be assumed. For example, the assumption thatf2 andf4 are each fixed at a value of 0.5 might reduce correlation. The five remaining unknowns in the regression analysis would then be tl,%, t3, t4, andff. In practice one first assumes the simplest possible model, then makes it more complex until correlation sets in, or until the mean squared error fails to decrease significantly. 406

...


Chapter 7

Polarization Measurement

Manual null ellipsomerry is accurate but infrequently done, due to the length of time needed to acquire sufficient data for any meaningful materids analysis. Automated null ellipsometers are used, for example, in the infrared, but are still slow. Numerous versions of kt automated ellipsometers have been built. 1-3 Examples are: 1 Polarization modulation

z Rotating analyzer Rotating polarizer. The most common versions are 2 and 3, and the rotating analyzer system will be briefly described here." Such a system consists of a light source, monochromator, collimating optics, and polarizer preceding the sample of Figure 1, and a rotating polarizer (called the analyzer) and detector following the sample. The intensity of the light measured at the detector oscillates sinusoidally according to the relation

3

I = 1 + acos2d+ PsinZA where a and p are the Fourier coefficients, and A is the azimuthal angle between the analyzer "fast axis" and the plane of incidence. There is a direct mathematical relationship between the Fourier coefficients and the Y and A ellipsometric parameters. The actual experiment involves recording the relative light intensity versus A in a computer. The coefficients 01 and p, and thus Y and A, can then be determined. By changing the angle of incidence and wavelength, the user can determine N sets of Y j and Ai values for the regression analysis used to derive the unknown physical properties of the sample. The polarizer and analyzer azimuthal angles relative to the plane of incidence must be calibrated. A procedure for doing this is based on the minimum of signal that is observed when the fist axes of two polarizers are perpendicular to each other. For details the reader can consult the literature.l1 Applications

In this section we will give some representative examples. Figure 4 shows the regression procedure for tan Y for the glass/Ti02/Ag/Ti02 system. The unknowns of the fit were the three thicknesses: TiO2, Ag,and the top TiO2. Initial guesses at the thicknesses were reasonable but not exact. The final thicknesses were 33.3 nm, 11.3 nm, and 26.9 nm, and the fits between measured 'Pi" and Aim and calculated (from Fresnel equations) Y/ and A/ were excellent. This means that the assumed optical constants and structure for the material were reasonable. Because Y and A can be calculated for any structure (no matter how complex, as long as planar parallel interhces are present), then the user can do predictive modeling. Figure 5 shows the expected A versus wavelength and angle of incidence for a

7.3

VASE

407

Glass/TiOz /Ag/TiOz Oe9

I

0.8

3 C

0.6

0

4-a

0.5

0.3

Figure 4

Data plus iterations 1,2, and 7 in regression analysis (data fit) for the optical coating glass/Ti02 / Ag / Ti02

Figure 5

Three-dimensional plot of predicted ellipsometric parameter data versus angle of incidence and wavelength.

structure with a GaAs substrate/50 nm of&&a~,~As/30 nm of GaAs/3 nm of oxide5 The best data are taken when A is near 90°, and generated surhces such as Figure 5 help enormously in finding the proper wavelength and angle regions to take data.& Equally u s d are contour plots made from the surfgces of Figure 5 which show quantitativelywhere the 90" f 20° regions of A will be found.*. l2 408


Chapter 7

Many materials have been studied; examples include: Dielectrics and optical coatings: Si3N4, Si02, SiOJV,,, Al2O3, a-C:H, ZnO, Ti02,ZnO/Ag/ZnOYTiOz/Ag/TiO2, Ago,In(Sn)203, and organic dyes. Semiconductorsand heterostructures: Si, poly-Si, amorphous Si, G A , ,41xGl-&, In,Gal-&, and numerous 11-VI and 111-V category compound semiconductors; ion implanted compound heterostructures, superlattices, and heterostructures exhibiting Franz-Keldysh oscillations. Work has been done on rhese materials at room temperature, as well as from cryogenic (4K) to crystal growth temperatures (900 K). Surface modifications and surface roughness: Cu, Mo, and Be laser mirrors; atomic oxygen modified (corroded) surfaces and films, and chemically etched surfaces. Magneto-optic and magnetic disc materials: DyCo, TbFeCo, garnets, sputtered magnetic media (CoNiCr alloys and their carbon overcoats). Electrochemical and biological and medical systems. In-situ measurements into vacuum systems: In these experiments the light beams enter and leave via optical ports (usually at a 70" or 75" angle of incidence), and w and A are monitored in time. Example studies include the measurement of optical constants at high temperatures, surface oxide formation and sublimation, surface roughness, crystal growth, and film deposition. In-situ measurements were recently reviewed by C o l l i n ~ . ~ Conclusions

Ellipsometry is a powerful technique for surface, thin-film, and interface analysis. It is totally nondestructive and rapid, and has monolayer resolution. It can be performed in any atmosphere including high-vacuum, air, and aqueous environments. Its principal uses are to determine thicknesses of thin films, optical constants of bulk and thin-film materials, constituent fractions (including void fractions) in deposited or grown materials, and surface and interfacial roughness. Recent trends in the relatively small community of scientists using ellipsometry in research have been towards in-situ measurements during crystal growth or material deposition or processing. Fast-acquisition automated ellipsometers have not been used widely in medical research, which represents an opportunity. Simple one-wavelength ellipsometers are in common use (and misuse due to correlated variables) in semiconductor processing. Use of a full spectroscopic ellipsometer is strongly advised. The ellipsometer user will always get data; but unfortunately may not always know when the data or the results of analysis are correct. Improper optical alignment, bad calibration constants, reflection from the back surface of partially trans7.3

VASE

409

parent materials, as well as correlation of variables are all potential problems to be aware of. Ellipsometry is a powerful technique when used properly. The authors wish to recognize financial support under grants NAG 3-1 54 and NAG 3-95 from the NASA Lewis Research Center, Cleveland, Ohio. Related Articles in the Encyclopedia

MOKE

References

R M. A. Azzam and N.M. Bashara. Ellipsometryand Polarized Light. North Holland Press, New York, 1977. Classic book giving mathematical details of polarization in optics. 2 D. E. Aspnes. In: Handbook of Optical Constana of Solid. (E. Palik, ed.) Academic Press,Orlando, 1985. Description of use of ellipsometry to determine optical constants of solids. 3 R E. Collins. Rev. Sci. Ima. 61,2029, 1990. Recent review of in-situ ellipsometry, in considerabledepth. 4 I?G. Snyder, M. C. Rost, G. H. Bu-Abbud, J. A. Woollam, and S. A. Alterovitz. J ofAppl. Pbys. 60,3293, 1986. First use of computer drawn three-dimensional surfaces (in wavelength and angle of incidence space) for ellipsometric parameters and A and their sensitivities. 5 S. A. Alterovitz, J. A. Woollam, and I? G. Snyder. Solidstate Tech. 31,99, 1988. Review of use of variable-angle spectroscopicellipsometer (VASE) for semiconductors. 6 J. A. Woollam and I? G. Snyder. M a t 4 Sci. Eng. B5,279,1990. Recent review of application of VASE in materials analysis. 7 K. G. Merkel, I? G. Snyder, J. A. Woollam, S. A. Alterovitz, and A. K. Rai. Japanese/. App. Phys. 28, 1 1 18, 1989. Application ofVASE to complicated multilayer semiconductortransistor structu~s. 8 E. Hecht. Optics. Addison-Wesley, Reading, 1987. W d written and illustrated text on classical optics. 9 G. H. Bu-Abbud, N. M. Bashara, and J. A. WooUarn. Thin Solid Film. 138,27, 1986. Description of Marquardt algorithm and parameter sensitivity correlation in ellipsometry. i o D. E. Aspnes. Thin Solid Film. 89,249, 1982. A detailed review of effective medium theory and its use in studies of optical properties of solids. 1

410


Chapter 7

11

12

7.3

D. E. Aspnes and A. A. Studna. App. Optics. 14,220,1973. Details of a rotating analyzer ellipsometer design. W. A. McGahan, and J. A. Woollam. App. Pbys. Commun. 9, 1, 1989. Well written and illustrated review of electromagnetictheory applied to a multilayer structure including magnetic and magneto-optic layers.

VASE

41 1

V I B RAT10 NAL SPECTROSCOPIES AND NMR 8.1 Fourier Transform Infrared Spectroscopy, FTIR 416 8.2 Raman Spectroscopy 428 8.3 High-Resolution Electron Energy-Loss Spectroscopy, H E E L S 442 8.4 Solid State Nuclear Magnetic Resonance, NMR 460

8.0

INTRODUCTlON

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Energy-Loss Spectroscopy, H E E L S , uses electron impact. The fourth technique, Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. The most commonly used of these methods, and the most inexpensive, is FTIR In it a broad band source of IR radiation is reflected from the sample (or transmitted, for thin samples). The wavelengths at which absorption occurs are identified by measuring the change in intensity of the light after reflection (transmission) as a function of wavelength. These absorption wavelengths represent excitations of vibrations ofthe chemical bonds and are specific to the type of bond and the group of atoms involved in the vibration. IR spectroscopy as a method of quantitative chemical identification for species in solution, or liquids, has been commercially available for 50 years. The advent of fast Fourier transform methods in conjunction with interferometer wavelength detection schemes in the last 15 years has allowed 413

drastic improvement in resolution, sensitivity, and reliable quantification. During this time the method has become regularly used also for solids. The sensitivity toward different bonds (chemicalgroups) is extremely variable, going from zero (no coupling of the IR radiation to vibrational excitations because of dipole selection rules) to high enough to detect submonolayer quantities. Intensitiesand line shapes are also sensitive to local solid state effects, such as stress, strain, and defects (which can therefore be characterized), so quantification is difficult, but with suitable standards 5-1 0% accuracy in concentrations are achievable. The depth probed depends strongly on the material (whether it is transparent or opaque to IR radiation) and can be as little as 100A or as much as 1 mm. The chemical nature of opaque interfaces beneath transparent overlayers can therefore be studied. Grazing angle measurements greatly reduce the probing depth, restricting it to a monolayer for molecules absorbed on metal surfaces. Ofcen there is no spatial resolution (mm), but microfocus systems down to 20 pm exist. In Raman spectroscopy IR radiation of a single wavelength from a laser strikes the sample and the energy losses (gains) due to the Raman scattering process, which lead to some light being reemitted at lower (higher) frequencies, are determined. These loss (or gain) processes are again due to the coupling of the vibrational processes in the sample with the incident IR radiation. So, though the physics of the Raman process is quite different from that of IR spectroscopy (scattering instead of absorption), the information content is very similar. The selection rules defining which vibrational modes can be excited are different from IR, however, so Raman essentially provides complementary information. Cross sections for Raman scattering are extremely weak, resulting in Raman sensitivity being about a kctor of 10 lower than for FTIR However, better spatial resolution can be achieved (down to a few pm) because the single wavelength nature of the laser source allows an easy coupling to optical microscope elements. For the “fingerprinting identification of chemical composition not nearly so extensive a library of data is available as for IR spectroscopy. Because of this, and because instrumentation is generally more expensive, Raman spectroscopy is less widely used, except where the microfocus capabilities are important or where differences in selection rules are critical. Both IR and Raman have the great practical advantage of working in ambient atmosphere, and one can even study interfices through liquids. The third vibrational technique discussed here, HEELS, requires ultrahigh vacuum conditions. A monochromatic, low-energy electron beam (a few ev) is reflected from a sample surface, losing energy by exciting vibrations (cf., Raman scattering) as it does so. Since the reflected part of the beam does not penetrate the surface, the vibrational information obtained relates only to the outermost layers. Actually two separate scattering mechanisms occur. Scattering in the specular direction is a long-range dipole process that has the same selection rules as for I R Impact scattering is short range and nonspecular. It is an order of magnitude weaker than dipole scattering and has relaxed selection rules. Taking data in both the specular and off-specular 414

VIBRATIONAL SPECTROSCOPIES

...

Chapter 8

directions therefore maximizes the amount of information obtainable. The wavelength range accessible is wider in HREELS than in IR spectroscopy, but the resolution is orders of magnitude poorer, leading to overlapped vibrational peaks and little detailed information on individual line shapes. The major uses of HREELS have been identifying chemical species, adsorption sites, and adsorption geometries (symmetry) for monolayer adsorption at single crystal surfaces. For non-single crystal surfaces the energy-loss intensities are drastically reduced, but the technique is still usehl. It has been quite extensively used for characterizing polymer surfaces. For insulators charging can sometimes be a problem. The last technique discussed here, NMR, involves immersing the sample in a strong magnetic field (1-12 Tesla), thereby splitting the degeneracy of the spin states of those nuclei that have either an odd mass or odd atomic number and hence possess a permanent magnetic moment. About half the elements in the periodic table have isotopes fulfilling these conditions. Excitation between these magnetic levels is then performed by absorption of radiofrequency (RF) radiation. By measuring the energy at which the absorptions occur (the “resonance” energies) the energy differences between the spin (magnetic) states are determined. For any given magnetic field the values are element specific, but the nuclear magnetic moments and electronic environment surrounding the target atoms also exert an influence, splitting the absorption resonances into multiple lines and shifting peak positions. From these effects the local environment of the atoms concerned-the coordination number, local symmetries, the nature of neighboring chemical groups, and bond distances-can be studied. H-atom NMR has been used as an analytical tool for molecules in liquids for about 40 years to identify chemical groupings, and the sequence of groupings containing H atoms. It is also, of course, the basis of Magnetic Resonance Imaging, MRI, which is used medically. In the solid state, crystalline phases can be identified, and quantitative analysis can be achieved directly in mixtures from the relative intensities of peaks and the use of well-defined model compound standards. In many cases the NMK spectra of solids are rather broad and unresolved due to strong anisotropic effects with respect to the applied magnetic field. There are a number of ways of removing these effects, the most popular being magic-angle spinning of the sample, which can collapse broad powder patterns into sharp resonances that can be easily assigned. NMR is intrinsically a bulk technique; the signal comes from the entire sample which is immersed in the magnetic field. At least 10 mg of material is required (powders, thin films, or crystals), and to get any information specific to surfaces or interfaces requires large surface areas (10-150 m2/gm). Costs vary a lot ($200,000 to $1,200,000), depending on how wide a range of elements needs to be accessed, since this determines the range and magnitude of the magnetic fields and RF capabilities required.

415

8.1

FTlR Fourier Transform Infrared Spectroscopy J . N E A L COX

Contents

Introduction Basic Principles Methodologies and Accessories Interferences and Artifacts 9 Conclusions

9

Introduction

The physical principles underlying infrared spectroscopy have been appreciated for more than a century. As one of the fav techniques that can provide information about the chemical bonding in a material, it is particularly usem for the nondestructive analysis of solids and thin films, for which there are few alternative methods. Liquids and gases are also commonly studied, more often in conjunction with other techniques. Chemical bonds vary widely in their sensitivity to probing by infrared techniques. For example, carbon-sulfur bonds often give no infrared signal, and so cannot be detected at any concentration, while silicon-oxygen bonds can produce signals intense enough to be detected when probing submonolayer quantities, or on the order of 1013bonddcc. Thus, the potential utility of infrared spectrophotometry (IR) is a function of the chemical bond of interest, rather than being applicableas a generic probe. For quantitative analysis, modern instrumentation can provide a measurement repeatability of better than 0.1%. Accuracy and precision, however, are more commonly on the order of 5.0% (30), relative. The limitations arise h m sample-to-sample variations that modlfl the optical quality of the material. This causes slight, complex distortions of the spectrum that are dif416

VIBRATIONAL SPECTROSCOPIES ...

Chapter 8

ficult to eliminate. Sensitivity of the sample to environmental influences that modify the chemical bonding and the need to calibrate the infrared spectral data to reference methods-such as neutron activation, gravimetry, and wet chemistryalso tend to degrade slightly the measurement for quantitative work. The goal of the basic infrared experiment is to determine changes in the intensity of a beam of infrared radiation as a function of wavelength or frequency (2.550 pm or 4000-200 cm-', respectively) after it interacts with the sample. The centerpiece of most equipment configurations is the infrared spectrophotometer. Its function is to disperse the light from a broadband infrared source and to measure its intensity at each frequency. The ratio of the intensity before and after the light interacts with the sample is determined. The plot of this ratio versus frequency is the infrared spectrum. As technology has progressed over the last 50 years, the infrared spectrophotometer has passed through two major stages of development. These phases have significantly impacted how infrared spectroscopy has been used to study materials. Driven in part by the needs of the petroleum industry, the first commercial infrared spectrophotometers became available in the 1940s.The instruments developed at that time are referred to as spatially dispersive (sometimes shortened to dispersive) instruments because ruled gratings were used to disperse spatially the broadband light into its spectral components. Many such instruments are still being built today. While somewhat limited in their ability to provide quantitative data, these dispersive instruments are valued for providing qualitative chemical identification of materials at a low cost. The 1970switnessed the second phase of development. A new (albeit much more expensive) type of spectrophotometer, which incorporated a Michelson interferometer as the dispersing element, gained increasing acceptance. All frequencies emitted by the interferometer follow the same optical path, but differ in the time at which they are emitted. Thus these systems are referred to as being temporally dispersive.Since the intensity-time output of the interferometer must be subjected to a Fourier transform to convert it to the familiar infrared spectrum (intensity-frequency), these new units were termed Fourier Transform Infrared spectrophotometers, (FTIR). Signal-to-noise ratios that are higher by orders of magnitude, much better resolution, superior wavelength accuracy, and significantly shorter data acquisition times are gained by switching to an interferometer. This had been recognized fbr several decades, but commercialization of the equipment had to await the arrival of local computer systemswith significant amounts of cheap memory, advances in equipment interfacing technology, and developments in fasr Fourier-transform algorithms and circuitry. Beyond the complexities of the dispersive element, the equipment requirements of infrared instrumentation are quite simple. The optical path is normally under a purge of dry nitrogen at atmospheric pressure; thus, no complicated vacuum pumps, chambers, or seals are needed. The infrared light source can be cooled by water. No high-voltage connections are required. A variety of detectors are avail8.1

FTlR

417

able, with deuterated tri-glycene sulfite (DTGS) detectors offering a good signalto-noise ratio and linearitywhen operated at room temperature. For more demanding applications, the mercury cadmium telluride (HgCdTe, or mer-cad telluride) detector, cooled by liquid nitrogen, can be used for a kctor-of-ten gain in sensitivity. With the advent of FTIR instrumentation, IR has experienced a dramatic increase in applicationssince the 1970s, especiallyin the area of quantitative analysis. FTIR spectrophotometry has grown to dominate the field of i n h e d spectroscopy. Experiments in microanalysis, surface chemistry, and ultra-thin films are now much more routine. The same is true for interfaces, if the infrared characteristicsof the exterior layers are suitable. While infrared methods still are rarely used to profile composition as a function of depth, microprobing techniques available with FTIR technology permit the examination of micropartides and vprofiling with a spatial resolution down to 20 pm. Concurrent with opening the field to new areas of research, the high level of computer integration, coupled with robust and nondestructive equipment configurations,has accelerated the move of the instrument out of the laboratory. Examples are in VLSI, computer-disk, and chemicals manukcturing, where it is used as a tool for thin-film, surfice coating, and bulk monitors. Unambiguous chemical identification usually requires the use of other techniques in conjunctionwith IR. For gases and liquids, Mass Spectrometry (MS) and Nuclear Magnetic Resonance Spectrometry (NMR) are routinely employed. The former, requiring only trace quantities of material, determines the masses of the molecule and of characteristic fragments, which can be used to deduce the most likely structure. MS data is sometimes supplemented with infrared results to distinguish certain chemical configurations that might produce similar fragment patterns. NMR generally requires a few milliliters of sample, more than needed by either the FTIR or MS techniques, and can identify chemical bonds that are associated with certain elements, bonds that are adjacent to each other, and their relative concentrations. Solids can also be studied by these methods. For MS, the sensitivity remains high, but the method is destructive because the solid must somehow be vaporized. While nondestructive, the sensitivity of NMR spectrometry is typically much lower for direct measurements on solids; otherwise, the solid may be taken into solution and analyzed. For thin films, both the MS and NMR methods are destructive. Complementary data for surfaces, interfaces, and thin films can be obtained by techniques like X-ray photoelectron spectroscopy, static secondary ion mass spectrometry, and electron energy loss spectrometry. These methods probe only the top few nanometers of the material. Depending upon the sample and the experimental configuration, IR may be used as either a surface or a bulk probe for thin films. For surfice analysis, FTIR is about a factor of 10 less sensitive than these alternative methods. Raman spectroscopy is an optical technique that is complementary to infrared methods and also detects the vibrational motion of chemical bonds. While able to achieve submicron spatial resolution, the sensitivity of the 418


Chapter 8

Raman technique is usually more than an order of magnitude less than that of FTIR As a surfice probe, FTIR works best when the goal is to study a thin layer of material on a dissimilar substrate. Lubricating oil on a metal surface and thin oxide layers on semiconductor surfaces are examples. FTIR techniques become more difficult to apply when the goal is to examine a surface or layer on a similar substrate. An example would be the study of thin skins or surface layers formed during the curing cycles used for photoresist or other organic thin films deposited from the liquid phase. If the curing causes major changes in the bulk and the surface, FTIR methods usually cannot discriminate between them, because the beam probes deeply into the bulk material. The limitations as a surface probe often are dictated by the type of substrate being used. A metal or high refractive-index substrate will reflect enough light to permit sensitive probing of the surface region. A low refractive index substrate, in contrast, will permit the beam to probe deeply into the bulk, degrading the sensitivity to the surface. The discussions presented in this article pertain to applications of FTIR, because most of the recent developments in the field have been attendant to FTIR technology. In many respects, FTIR is a “science of accessories”. A myriad of sample holders, designed to permit the infrared light to interact with a given type of sample in an appropriate manner, are interfaced to the spectrophotometer.A large variety of “hyphenated” techniques, such as GC-FTIR (gas chromatography-FTIR) and TGA-FTIR (thermo-gravimetric analysis-FTIR), also are used. In these cases, the effluent emitted by the GC, TGA, or other unit is directed into the FTIR system for time-dependent study. Hyphenated methods will not be discussed further here. Still, common to all of these methods is the goal of obtaining and analyzing an infrared spectrum. Basic Principles Infrared Spectrum

Define Io to be the intensity of the light incident upon the sample and 1to be the intensity of the beam after it has interacted with the sample. The goal of the basic infrared experiment is to determine the intensity ratio 1/10 as a function of the frequency of the light (w). A plot of this ratio versus the frequency is the infrared spectrum. The infrared spectrum is commonly plotted in one of three formats: as transmittance, reflectance, or absorbance. If one is measuring the fraction of light transmitted through the sample, this ratio is defined as

L’ 8.1

(2)

FTlR

tu

419

A

.a73

-

B .15-

a

:.m.018

C

-

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I

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I

1466

1935

1199

I 1066

I 992

I

I

I

799

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532

I 999

WAVENUHEER

Figure 1

The FTIR spectrum of the oxide of silicon (thin film deposited by CVD). Primary features: (a), asymmetric stretching mode of vibration; (b), bending mode of vibration; IC), rocking mode of vibration.

where T, is the transmittance of the sample at frequency w, and It is the intensity of the transmitted light. Similarly, if one is measuring the light reflected from the surface of the sample, then the ratio is equated to &, or the reflectance of the spectrum, with It being replaced with the intensity of the reflected light I,. The third format, absorbance, is related to transmittance by the Beer-Lambert Law:

where c is the concentration of chemical bonds responsible for the absorption of infrared radiation, b is the sample thickness, and. E, is the frequency-dependent absorptivity, a proportionality constant that must be experimentally determined at each w by measuring the absorbance of samples with known values of bc. As a firstorder approximation the Beer-Lambert Law provides an simple foundation for quantitating FTlR spectra. For this reason, it is easier to obtain quantitative results if one collects an absorbance spectrum, as opposed to a reflectance spectrum. Prior to the introduction of FTIR spectrophotometers, infrared spectra were usually published in the transmittance format, because the goal of the experiment was to obtain qualitative information. With the growing use of FTIR technology, a quantitative result is more often the goal. Today the absorbance format dominates, because to first order it is a linear function of concentration. Qualitative and Quantitative Analysis

Figure 1 shows a segment of the FTIR absorbance spectrum of a thin film of the oxide of silicon deposited by chemical vapor deposition techniques. In this film, sil420


Chapter 8

mfi 1340

1291

1242

1195

1144

1m5

1046

997

948

WAVENUMBER

Figure 2

Spectral parameters typically used in band shape analysis of an m R spectrum: peak position, integrated peak area, and FWHM.

icon is tetrahedrally coordinated with four bridging oxygen atoms. Even though the bond angles are distorted slightly to produce the random glassy structure, this spectrum is quite similar to that obtained from crystalline quartz, because most features in the FTIR spectrum are the result of nearest neighbor interactions. In crystalline materials the many vibrational modes can be classified by the symmetry of their motions and, while not rigorous, these assignments can be applied to the glassy material, as well. Thus the peak near 1065 cm-' arises from the asymmetric stretching motions of the Si and 0 atoms relative to each other. The band near 8 15 cm-' arises from bending motions, while the one near 420 cm-' comes from a collective rocking motion. These are not the only vibrational modes for the glass, but they are the only ones that generate electric dipoles that are effective in coupling with the electromagnetic field. For example, the glass also has a symmetric stretching mode, but since it generates no net dipole, no absorption band appears in the FTIR spectrum. For more quantitative work, the hndamental theory of infrared spectroscopy delineates a band shape analysis illustrated in Figure 2. Three characteristics are commonly examined: peak position, integrated peak intensity, and peak width. Peak position is most commonly exploited for qualitative identification, because each chemical functional group displays peaks at a unique set of characteristic frequencies. The starting point for such a functional-group analysis is a table or computer database of peak positions and some relative intensity information. This provides a fingerprint that can be used to identify chemical groups. Thus the three peaks just described for the oxide of silicon can be used to identify that material. Typical of organic materials, C-H bonds have stretching modes around 3200 an-'; C = 0,around 1700 cm-'. Thus, the composition of oils may be qualitatively identified by classifying these and other peak positions observed in the spectrum. In 8.1

FTIR

42 1

addition, some bands have positions that are sensitive to physicomechanicalproperties. As a result, applied and internal pressures, stresses, and bond strain due to swelling can be studied. The Beer-Lambert Law of Equation (2) is a simplification of the analysis of the second-band shape characteristic, the integrated peak intensity. If a band arises from a particular vibrational mode, then to the first order the integrated intensity is proportional to the concentration of absorbing bonds. When one assumes that the area is proportional to the peak intensity, Equation (2) applies. In solids and liquids, peak width-the third characteristic-is a function of the homogeneity of the chemical bonding. For the most part, factors like defects and bond strain are the major sources of band width, usually expressed as the full width at half maximum (FWHM). This is due to the small changes these fictors cause in the strengths of the chemical bonds. Small shifts in bond strengths cause small shifts in peak positions. The net result is a broadening of the absorption band. The effect of curing a material can be observed by peak-width analysis. As one anneals defects the bands become narrower and more intense (to conserve area, if no bonds are created or destroyed). Beyond the standard analysis, higher order band shape properties may also be examined, such as peak asymmetry. For example, the apparent shoulder on the high-frequency side of the band in Figure 2 may be due to a second band that overlaps the more prominent feature. For many applications, quantitative band shape analysis is difficult to apply. Bands may be numerous or may overlap, the optical transmission properties of the film or host matrix may distort features, and features may be indistinct. If one can prepare samples of known properties and collect the FTIR spectra, then it is possible to produce a calibration matrix that can be used to assist in predicting these properties in unknown samples. Statistical, chemometric techniques, such as PLS (partial least-squares) and PCR (principle components of regression), may be applied to this matrix. Chemometric methods permit much larger segments of the spectra to be comprehended in developing an analysis model than is usually the case for simple band shape analyses. Methodologiesand Accessories

A large number of methods and accessories have been developed to permit the infrared source to interact with the sample in appropriate ways. Some of the more common approaches are listed below. Single-pass transmission

The direct transmission experiment is the most elegant and yields the most quantifiable results. The beam makes a single pass through the sample before reaching the detector. The bands of interest in the absorbance spectrum should have peak absorbances in the range of 0.1-2.0 for routine work, although much weaker or stronger bands can be studied. Various holders, pellet presses, and liquid cells have been 422


Chapter 8

developed to permit samples to be prepared with the appropriate path length. Diamond anvil c e h permit pliable samples to be squeezed to extremely thin path lengths or to be studied under applied pressures. Long path-length cells are used for samples in the gas phase. Thin films and prepared surfaces can be studied by transmission if the supporting substrate is transparent to the infrared. The highest sensitivity is obtained with double-beam or pseudo-double-beam experimental configurations. An example of the latter is the interhf experiment, where a single beam is used, but a sample and reference are alternately shuttled into and out of the beam path. Reflection

If the sample is inappropriate for a transmission experiment, for instance if the supporting substrate is opaque, a reflectance configuration will ofien be employed. Accessories to permit specular, diffuse, variable-angle, and grazing-incidenceexperiments are available. The angle of incidence can be adjusted to minimize multiplereflection interferences by working at the Bragg angle for thin films, or to enhance the sensitivity of the probe to surface layers. A subset of this technique, Reflection Absorption (RA) spectroscopy, is capable of detecting submonolayer quantities of materials on metal surfaces. These grazing and RA techniques can enhance surfice sensitivities by using geometries that optimize the coupling of the electromagnetic field at the metal surface. In some instances it has been possible to deduce preferred molecular orientations of ordered monolayers. Attenuated Total Reflection

In this configuration an Attenuated Total Reflection (ATR) crystal is used, illustrated in Figure 3. The infrared beam is directed into the crystal. Exploiting the principles of a waveguide, the change in refractive index at the crystal surface causes the beam to be back-reflected several times as it propagates down the length of the crystal before it finally exits to the detector. If the sample is put into contact with the crystal surface, the beam will interact weakly with the sample at several points. For extremely thin samples, this is a means of increasing the effective path length. Since the propagating beam in the crystal barely penetrates through the surface of the sample adjacent to the crystal, signals at a sample surface can be enhanced, as well. This also helps in the study of opaque samples. Approximately fivefold amplificarions in signals are typical over a direct transmission experiment. The quality of the crystal-sample interface is critical, and variability in that interface can make ATR results very difficult to quantify. Emission

When samples are heated, they emit infrared radiation with a characteristic spectrum. The IR emission of ceramics, coals, and other complicated solids and thin films can be studied. Also, if conditions make it difficult to use an infrared source 8.1

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423

I

10

Figure3

Typical beam path configuration for collecting an FTIR spectrum using an attenuated total refleetance element: is the incident infrared beam, I is the exiting beam.

(such as an in situ measurement of a thin film in the deposition chamber) but permit the controlled heating of a sample, then emission methods provide a means of examining these materials. Microscopy

Infrared microscopes can focus the beam down to a 20-prn spot size for microprobing in either the transmission or reflection mode. Trace analysis, microparticle analysis, and spatial profiling can be performed routinely. tnterferencesand Artifacts

Equipment technology and processing s o h a r e for FTIR are very robust and provide a high degree of reliability. Concerns arise for only the most demanding applications. For quantitative work on an isolated feature in the spectrum, the rule of c h u b is that the spectrometer resolution be one-tenth the width of the band. FTIR instruments routinely meet that requirement for solids. Short- and long-term drift in the spectral output can be caused by several factors: drift in the output of the infrared light source or of the electronics, aging of the beam splitter, and changes in the levels of contaminants (water, COZ, etc.) in the optical path. These problems are normally eliminated by rapid, routine calibration procedures. The two complicating hctors that are encountered most frequently are the linearity of detector response and stray light scattering at low signal levels. DTGS 424


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Chapter 8

detectors are quite linear and reliable, while MCT detectors can saturate at relatively low light levels. Stray light can make its way to the detector and be erroneously detected as signal, or it can be backscattered into the interferometer and degrade its output. A problem only at very low signal levels or with very reflective surfaces, proper procedures can minimize these effects. Intrinsic or Matrix

Few cross sections for infrared absorption transitions have been published and typically they are not broadly applicable. The strength of the absorption depends upon changes in the dipole moment of the material during the vibrational motion of the constituent atoms. However, these moments are also very sensitive to environmental factors, such as nearest neighbor effects, that can cause marked changes in the infrared spectrum. For example, carbon-halogen bonds have a stretching mode that may be driven from a being very prominent feature to being an undetectable feature in the spectrum by adding electron-donating or -withdrawing substituents as nearest neighbors. For carell quantitative work, then, model compounds that are closely representative of the material in question are often needed for calibration. Interface Optical Effeck

For thin-film samples, abrupt changes in refractive indices at interfaces give rise to several complicated muftipi2reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quanti_tative work in thin films. Note, however, that these interferences are hnctions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useid information beyond that of chemical bonding potentially may be extracted from the FTIR spectra. Many materials have grain boundaries or other microstructural features on the order of a micrometer or greater. This.is on the same scale as the wavelength of the infrared radiation, and so artifacts due to the wavelength-dependent scattering of light at these boundaries can be introduced into the spectra, Thin films, powders, and solids with rough surfices are the most affkcted. Again these artifacts are dEicult to realistically model, but properly analyzed can provide additional information about the sample. Conclusions

The principles of infrared spectroscopy can be exploited to extract information on the chemical bonding of an extremely wide variety of materials. The greatest strength of the technique is as a nondestructive, bulk probe of glassy and amor-

8.1

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425

phous materials, where few alternate methods exist for obtaining chemical information. For other materials, FTIR is a valuable member in the arsenal of characterization tools. Other methods that are most likely to provide similar information include rarnan spectroscopy, X-ray photoelectron spectroscopy, NMR, MS, SIMS, and high-resolution electron energy-loss spectroscopy. The nondestructive, noninvasive potential of the infrared technique, and its ease of use, continues to distinguish it from these other methods, with the exception of Raman spectroscopy. The trends begun with the general introduction of FTIR technology will undoubtedly continue. It is safe to say that the quality of the data being produced far exceeds our ability to analyze it. In fact, for many current applications, the principle limitations are not with the equipment, but rather with the quality of the samples. Thus, the shift from qualitative to quantitative work will proceed, reaching high levels of sophisticationto address the optical and matrix interference problems discussed above. With extensive computerization, the ease of use, and the robustness of equipment, movement of the instrumentationfrom the research laboratory to the manufacturing environment, for application as in situ and at-line monitors, will continue. In situ work in the research laboratory will also grow. New environments for application appear every day and improved computer-based data processing techniques make the rapid analysis of large sets of data more commonplace. These developments, coupled with rapid data acquisition times, are making possible the timely evaluation of the results of large-scale experiments. Most likely, much of the new physicochemical information developed by applying FTIR technology will come from trends observed in detailed studies of these large sample data sets. Related Articles in the Encyclopedia

Raman Spectroscopy and H E E L S References 1

2

3

B. George and P McIntyre. Analytical Chemistry 6y Open Learning: Infiared Spectroscopy. John Wiley & Sons, New York, 1987. A good primer on the basics of applied infrared spectroscopy. I?R. Griffiths and J. A. de Haseth. Fourier TransfirmInfiaredspectrometry. John Wiley & Sons, New York, 1986. Chapters 1-8 review FTIR equipment in considerable detail. Chapters 9-1 9 describe applications, including surfice techniques (Chapter 17). h t i c a l Fourier TransfirmInfiared Spectroscopy: Idustrial and Laboratory ChemicalAna&sis.(J.R. Ferraro and IC Krishnan, eds.) Academic Press, New York, 1990. Chapters 3 (by K. Krishnan and S. L. Hill) and

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4

5

8.1

Chapter 7 (by H. Ishida and A. Ishitani) review microscopicand surf$ce analytical techniques. Chapter 8 (by D. M. Haaland) reviews developments in statisticalchemometricshr data analysis. 0.S. Heavens. OpticalProperticsofThin SoldFiLm. Butterworths, 1955. Chapter 4 presents a detailed mathematical descriptionof the Fresnel fringingphenomenon for the transmission of light through thin films. C. E Bohren and D. R Huffinan. Absorption andScaming offight by SmalPartich. JohnWdey & Sons, New York, 1983. Parts 1 and 2 describe the theory of the scatteringproblem in some detail. Part 3 compares theory with experiment.

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8.2

Raman Spectroscopy WILLIAM B. WHITE

Contents

Introduction Basic Principles Instrumentation Sample Requirements Bulk Raman SpectroscopicAnalysis Microfbcus Raman SpectroscopicAnalysis Thin and Thick Films Conclusions

Introduction

To a surprisingly accurate approximation, molecules and crystals can be thought of as systems of balls (atoms) connected by springs (chemical bonds). These systems can be set into vibration, and vibrate with frequencies determined by the mass of the balls (atomic weights) and by the stiffnessof the springs (bond force constants). Diatomic molecules ( 0 2 , COYHC1, etc.) having two balls connected by a single spring have only one fundamental vibrational frequency. The same is true for diatomic crystals, which have a single diatomic formula unit in the primitive unit cell (NaC1, ZnS, diamond, etc.), although the details are more complicated. The number of possible vibrational motions is 3n-6 for nonlinear molecules and 3n- 3 for crystals, where n is the number of atoms in the molecule or in the primitive unit cell of the crystal. The mechanical molecular and crystal vibrations are at very high frequencies, in the range 1012-10'4 Hz (3-300 Fmwavelength),which places them in the infrared (IR) region of the electromagnetic spectrum. Coupling between incident infrared radiation and the electronic structure of the chemical bond produces the infrared absorption spectrum as a direct means of observing molecular 428


Chapter 8

,

PELLA-BROC PRISM

n

CZI ANALYZER

/

m 0

Figure 1

I

Drawing of single-channel Raman spectrometer showing Czerny-Turner type double monochromator. Collecting optics for scattered beam are not shown.

and crystal vibrations (see the article on FTIR). The Raman spectrum arises from an indirect coupling of high-frequency radiation (usually visible light, but also ultraviolet and near infrared) with the electron clouds that make up the chemical bonds. Thus, although both IR absorption and Raman spectroscopy measure the vibrational spectra of materials, the physical processes are different, the selection rules that determine which of the vibrational modes are excited are different, and the two spectroscopies must be considered to be complementary. Both, however, are characterization probes that are sensitive to the details of atomic arrangement and chemical bonding. Raman spectroscopy is primarily a structural characterization tool. The spectrum is more sensitive to the lengths, strengths, and arrangement of bonds in a material than it is to the chemical composition. The Raman spectrum of crystals likewise responds more to details of defects and disorder than to trace impurities and related chemical imperfections. Basic Principles

The essentials of the Raman scattering experiment are shown in Figure 1. An intense monochromatic light beam impinges on the sample. The electric field of the incident radiation distorts the electron clouds that make up the chemical bonds in the sample, storing some energy. When the field reverses as the wave passes, the 8.2

Raman Spectroscopy

429

ANTI -STOKES SCATTERING

STOKES SCATTERING

a LASER

-21,000

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-1500

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18,000

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distorted electron clouds relax and the stored energy is reradiated. Although the incident beam may be polarized so that the electric field is oriented in a specific direction with respect to the sample, the scattered beam is reradiated in all directions, making possible a variety of scattering geometries. Most of the stored energy is reradiated at the same frequency as that of the incident exciting light. This component is known as the Rayleigh scattering and gives a strong central line in the scattering spectrum (Figure 2). However, a small portion of the stored energy is transferred to the sample itself, exciting the vibrational modes. The vibrational energies are deducted from the energy of the incident beam and weak side bands appear in the spectrum at frequencies less than that of the incident beam. These are the Raman lines. Their separation from the Rayleigh line is a direct measure of the vibrational frequencies of the sample. The reverse process also occurs. Existing vibrations that have been excited by thermal processes, can be annihilated by coupling with the incident beam and can add their energies to that of the source. These appear as side bands at higher wavenumbers. The Raman process that excites molecular and crystal vibrations is called Stokes scattering; the process that annihilates existing vibrations is called antiStokes scattering. The two spectra are mirror images on opposite sides of the Ray430


Chapter 8

leigh line (Figure 2). However, because anti-Stokes scattering depends on the existence of thermally activated vibrations, the anti-Stokes intensities are strongly temperature dependent, whereas the Stokes intensities are only weakly temperature dependent. For this reason, anti-Stokes scattering is rarely measured, except for the specialized technique known as coherent anti-Stokes Raman spectroscopy.' Because the vibrational frequencies are measured by differences between the frequency of the Raman line and the Rayleigh line, most spectrometers are set up to display the difference frequency (wavenumber) directly, defining the exciting frequency as 0. The result sometimes causes conhion: as the displayed Raman wavenumber increases, the true wavenumber decreases. The Raman effect is extremely weak. Rayleigh scattering from optically transparent samples is on the order of 10-3-10-5 of the intensity of the exciting line. Raman scattering is from lW3 to lo4 of the intensity of the Rayleigh line. For this reason Raman spectroscopy prior to the 1960s was a highly specialized measurement carried out in a few laboratories mainly with mercury discharge lamp sources, fast prism spectrographs, and photographic plate detectors. In such equipment, the most intense Raman lines are at the threshold of visibility to the darkness-adapted human eye. The invention of continuous gas lasers, which provided the needed intense monochromatic source,and the invention in 1965 of the double monochromator system for stray light discrimination combined to make the modern Raman spectrometer possible.

Instrumentation Single-Channel Dispersive Instruments

The most widely used equipment for Raman spectroscopy follows the scheme shown in Figure 1. The source is usually a continuous gas laser. The 488-nm and 514.5-nm lines of the argon ion laser are commonly used, although argon has other, somewhat weaker lasing lines that can be used for special purposes. He-Ne lasers (632.8 nm) and the red line of the ktypton laser (647.1 nm) also are used. The Raman scattering intensity decreases as the fourth power of the wavelength so that there is an advantage to using the shorter wavelength argon ion lines unless other circumstances, such as sample fluorescence, sample photodecomposition, or the location of the optical absorption edge in semiconductor materials, require a less energetic excitation source. The output from gas lasers is intrinsically polarized so that a polarization rotator is needed to control the polarization orientation with respect to the sample, especiallywith oriented single-crystal specimens. The sample holder is indicated by a cube in Figure 1 to show the 90" scattering arrangement. The part of the sample actually measured is a volume, typically 200500 pm on a side, where the laser beam and the scattered beam intersect. Omitted from the sketch are the focusing optics that collect the scattered iight and bring it to a focus on the entrance slit. Many sample mounting arrangements are possible, 8.2

Raman Spectroscopy

43 I

depending on the physical state of the sample. An analyzer in front of the entrance slit permits the determination of the polarization of the scattered beam. The commonly used monochromator is a Czerny-Turner grating type, usually of 1-m focal length, as sketched in Figure 1. This design features two collimating mirrors and a planar grating that is rotated to sweep the spectrum across the exit slit. Because Raman spectra are extremely weak, stray light within the monochromator must be effectively suppressed. The second monochromator, used as a stray light filter, reduces the stray light to the range, providing a clean background for the Raman spectra. Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. Diode Array and Charge-CoupledDetector Systems

A disadvantage of the single-channel scanning monochromator is that substantial times are needed to collect a spectrum, which is unsatisfictory for transient species, kinetic studies, and observation of unstable compounds. Linear diode array detectors allow the entire spectrum to be captured at one time, although resolution is limited by the number of pixels. Charge-coupled device detector arrays are now used on some commercial spectrometers. These are two-dimensional arrays of detector elements, 578 x 385 pixels being a common choice. Both diode array and charge-coupleddetectors permit the use of pulsed laser sourcesand provide infrared sensitivity. Nd-YAG lasers operating at a wavelength of 1.06 pm can then be used to eliminate many problems of fluorescenceand sample photodecomposition. Fourier TransformRaman Spectrometets

A relatively recent development is the adaptation of Fourier transform spectrometers commonly used for infrared absorption spectroscopy to Raman spectroscopy. Fourier transform Raman spectroscopy has the same advantages as other applications of interferometrictechniques. Elimination of the slits greatly increases energy throughput, an important advantage for weak Raman signals. The multiplex advantage permits more rapid accumulation of spectral data, although the alternative of diode array or charge-coupled detectors makes this advantage less important for Raman spectroscopy than for IR spectroscopy. Sample Requirements Liquids

Liquids and solutions can be measured in special cells that have optical windows at right angles, or they can be contained in capillary tubes or small vials. The latter are 432


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RefJected Loser

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Figure 3

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Scattering geometries appropriate to la1 liquids in capillaries or glass fibers; (b) powder pellets, solid slabs of ceramic or rock, or films on substratss; and IC) oriented single crystals.

mounted so that the laser passes along one diameter of the capillary while the scattered beam is along a perpendicular diameter (Figure 3). Powders and Po1yctystall;ne Sol;&

Usually, pamde size has relatively little &kct on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained fbm powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90"scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surfice but not into the entrance slit (Figure 3). Single-Crystal Measummenta

Maximum information is obtained by making Raman measurements on oriented, transparent single crystals. The essentials of the experiment are sketched in Figure 3. The crystal is aligned with the crystallographic axes parallel to a laboratory coordinate system defined by the directions of the laser beam and the scattered beam. A useful shorthand for describing the orientational relations (the Port0 notation) is illustrated in Figure 3 as z(xz)y. The first symbol is the direction of the laser beam; the second symbol is the polarization direction of the laser beam; the thiid symbol is the polarization direction ofthe scattered beam; and the fourth symbol is the direction ofthe scattered beam, all with respect to the laboratorycoordinatesystem. 8.2

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Spectra From Thick and Thin Films

Raman spectroscopy is especially suited for structural characterization of films and coatings. Both oblique (Figure 3b) and backscattering geometries can be used. The dective penetration depth of the laser depends on fdm transparency and on the scatteringgeometry but in most cases the spectrum of the substrate will be superimposed on the spectrum of the film and must be accounted for. The minimum observable film thickness varies with the intrinsic scattering power of the material but is in the range of a few pn for most materials. Resonance enhancement may decrease the minimum thickness. Useful data usually can be obtained from both crystalline and noncrystalline films. Sutface-Enhanced Raman Spectroscopy

Intensity enhancement takes place on rough silver surfaces. Under such conditions, Raman scattering can be measured from monolayers of molecular substances adsorbed on the silver (pyridine was the original test case), a technique known as surface-enhanced Raman spectro~copy.~ More recently it has been found that surhce enhancement also occurs when a thin layer of silver is sputtered onto a solid sample and the Raman scattering is observed through the silver. High-Temperature and High-pressure Raman Spectroscopy

Because Raman spectroscopyrequires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some difficulty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphirewindows for the laser and scattered beams. However, Raman spectroscopy is the characterization tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa.* Fluorescence, Sample Heating and Sample Photodecomposition

Although Raman spectroscopyis very versatile with regard to sample form, partide size, and composition, certain interferences occur. Many transparent solid samples contain impurities that fluoresce under excitation by the laser beam. Fluorescence bands are usually extremelybroad and are often of sufficient intensity to completely mask the weaker Raman scattering. Fluorescence can sometimes be avoided by using lower energy excitation, such as the red lines of H e N e or Krypton lasers, or by using a Nd-YAG laser with a diode array detector. If solid samples of interest are being synthesized for measurement, one can sometimes eliminate fluorescenceby deliberately doping the sample with luminescence poisons like ferrous iron. Nar434


Chapter 8

row-band fluorescence can be distinguished from Raman scattering by measuring the spectra using two different wavelength exciting lines (the 488-nm and 5 14.5-nm lines of argon, for example). Because the instrument wavenumber scale is set to 0 at the laser line, Raman lines will appear at the same wavenumber position under both excitations whereas fluorescence lines will appear to shift by the wavenumber difference between the two exciting lines. Because the laser beam is focused on the sample surface the laser power is dissipated in a very small area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shafc and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. Bulk Raman Spectroscopic Analysis Fingerprinting

Raman spectra of molecules and crystals are composed of a pattern of relatively sharp lines. The wavenumber scale for most vibrations extends from 50 cm-' to about 1800 cm-' with some molecular vibrations extending to 3500 cm-'. Line widths are on the order of 1-5 cm-'. The Raman spectrum thus has a fairly high density of information and can be used as a fingerprint for the identification of unknown materials by direct comparison of the spectrum of the unknown with spectra in a reference catalog. An example for a series of strontium titanates is shown in Figure 4. This approach is widely used for NMR spectra, X-ray diffraction powder patterns, and infrared spectra as well. As with these other techniques, the success of fingerprinting depends on the availability of a complete, high-quality catalog of reference spectra. Catalogs of reference Raman spectra are much less complete than the corresponding catalogs for IR, ultraviolet, and nuclear magnetic resonance spectra. Users of Raman spectroscopy should compile their own reference spectra. Crystar Spectra

From a knowledge of the crystal structure it is possible to calculate selection rules for each orientation position and thus gain considerable insight into the vibrational motions of the crystal. The interpretation of such spectra, which show a lot of detail, goes well beyond characterization application^.^ Ordered Structures and Phase Transitions

Raman spectroscopy is sensitive to ordering arrangements of crystal structures, the effect depending on the type of order. Ordering atoms onto specific lattice sites in 8.2

Raman Spectroscopy

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Figure 4

Raman spectra of a series of strontium titanates showing typical line shapes and information available for fingerprinting. The broadening of the highwavenumber lines is related to the polar character of the TiOBoctahedra that occur in all of these structures.

the parent structure often forms derivative structureshaving different space groups. The new symmetry leads to relaxed selection rules, which in turn lead to new Raman lines or to the splitting of the lines of the parent structure. Group theoretical calculations allow a good prediction of expected spectral behavior for possible ordering schemes, and Raman (IR) spectra can be used select the correct model. Some materials undergo transitions from one crystal structure to another as a function of temperature and pressure. Sets of Raman spectra, collected at various temperatures or pressures through the transition often provide usell information on the mechanism of the phase change: first or second order, order /disorder, sofi mode, etc. 436


Chapter 8

K2O .2Ge02

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Raman spectra of crystalline and glassy potassium digermanate showing comparison betweencrystal spectra and glass spectra.

Defects and Structural Disorder

Raman spectra of solid solutions, crystals with lattice defects, and systems having other types of structural disorder usually exhibit a pronounced line broadening in comparison with ordered structures. Structural defects, such as lattice vacancies, produce line broadening with little remperature dependence. Orientational disorder, arising fiom alternativepossible orientations of molecules in crystals, dipoles in highly polar crystals, and nonbonding lone pair electrons in ions like Pb2+and As3+, produce Raman lines that are broad at room temperature but become narrow as temperatures are lowered into the liquid nitrogen or liquid helium range. At high defect concentrations, greater than 1O-20% mole, the broadened Raman lines give way to a scattering continuum having little structure. Glasses and Gels

Raman spectroscopy is particularly useful for investigating the structure of noncrystalline solids. The vibrational spectra of noncrystalline solids exhibit broad bands centered at wavenumbers corresponding to the vibrational modes of the corresponding crystals (Figure 5). In silicate glasses shifts in the high-wavenumber bands 8.2

Raman Spectroscopy

437

is a measure of the degree of polymerization of the silicate network.6 The processes of nucleation and crystallization in glasses can be readily followed by the Raman spectrum because the sharp crystal bands are easily detected against the much weaker and broader glass bands. Gels are more disorganized than glasses and ofien have weaker Raman spectra. The processing steps from solution to sol, to gel, to glass in the sol-gel process of glass making can be followed in the Raman spectra. The loss of organic constituents of the gel can be followed as can the development of the bulk glass structure. Microfocus Raman Spectroscopic Analysis

By successllly marrying a Raman spectrometerto an optical microscope it is possible to obtain spectra having the resolution of optical microscopy-a few pm. The essential feature is a beam splitter through which the laser beam passes on its way to the microscope objective. The beam emerges hom the objective to strike a sample held on a standard microscope stage. Raman scattering is observed in the backscattering geometry. The scattered beam also passes through the beam splitter and reaches the entrance slit of the monochromator. Grain-by-GrainAnalysis and Crystal Zoning

The obvious application of microfocus Raman spectroscopyis the measurement of individual grains, inclusions, and grain boundary regions in polycrystallinematerials. No special surface preparation is needed. Data can be obtained from fresh fracture surfaces, cut and polished surfaces, or natural sudices. It is also possible to investigate growth zones and phase separated regions if these occur at a scale larger than the 1-2 pm optical focus limitation. With a special optical system at the sample chamber, combined with an imaging system at the detector end, it is possible to construct two-dimensional images of the sample displayed in the emission of a selected Raman line.7 By imaging from their characteristic Raman lines, it is possible to map individual phases in the multiphase sample; however, Raman images, unlike SEM and electron microprobe images, have not proved sufficiently useful to justify the substantial cost of imaging optical systems. Precipitates and the Liquid/ Solid Interface

With the microfocus instrument it is possible to combine the weak Raman scattering of liquid water with a water-immersion lens on the microscope and to determine spectra on precipitates in equilibrium with the mother liquor. Unique among characterization tools, Raman spectroscopywill give structural information on solids that are otherwise unstable when removed from their solutions.

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Fluid lnclusions

Natural crystals, synthetic crystals, and glasses often contain small bubbles that preserve samples of the fluid from which the crystals grew or of the atmosphere over the glass melt. Using a long focal length lens, the laser beam can be focused into inclusions at some depth below the crystal or glass surface. The Raman spectra then permit the identification of molecular species dissolved in the aqueous solutions or of components in the gas bubbles.* Grain Boundaries, Cracks, and Stressed Materials

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material; and by a carehl spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack.’ Thick and Thin Films

Because it is nondestructive, can be fine focused, and can be used in a backscattering geometry, Raman spectroscopy is a useful tool for characterizingfilms and layers either free standing or on various substrates. Semiconductors such as silicon, germanium and GaAs produce Raman spectra that give information on film crystallinity, the presence of impurity layers, and the presence of amorphous material. Films are ideal samples on the microscope stage of the microfocus instrument, and comparisons can easily be made between centers and margins of deposition zones. Raman spectroscopy has been applied to the characterization of chemical vapor deposition-grown diamond films, proving the formation of diamond and characterizing the presence of graphite and various amorphous carbons.lo The depth of film penetrated and sampled depends on the transparency of the sample to the laser radiation used. This is strongly dependent on the sample material and on the laser wavelength, and can vary from as little as a few pm to significanr fractions of a mm. If a film or coating is transparent, the film/substrate interhce and any other material deposited there is also accessible and may provide a detectable Raman signal. The absolute sensitivity of the Raman technique is routinely such that a film thickness of at least 0.1 pm is required to provide a workable signal. Special instrumentation, particularly high-sensitivity detectors, can reduce this to a few tens of nm and, as mentioned already, for surface-enhanced Raman scattering of molecules adsorbed on rough silver (and a few other) surfaces, submonolayer amounts are detectable. 8.2

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Conclusions

Raman spectroscopyis a very convenient technique for the identification of crysdline or molecular phases, for obtainingstructural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid-liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. In-situ Raman measurements will become more important in the future. In many types of vapor and liquid deposition systems, it is important to monitor the progress of reactions and changes in structure as films or crystals are grown. Fiber optic cables, sapphire light pipes and other optical systems are available for bringing the laser beam into the reaction chamber and for collecting the scattered light. Spectra can be collected without disturbing the deposition process. Related Artides in the Encyclopedia

FTIR and HREELS References 1

D. A. Long. Raman Spectroscopy. McGraw-Hill, New York, 1977. A standard reference work on Raman spectroscopywith much theoretical detail on the underlying physics. Most of the needed equations for any application of Raman spectroscopy can be found in this book

2

W. M. Tolles, J. W. Nibler, J. R. McDonald, and A. B. Harvey. Appl. Spectros. 31,253, 1977. R. K. Chang and T. E. Furtak; Suface Enhanced Raman Scamring. Plenum, New York, 1982. A collection of individually authored papers describing many different applications of SERS. R. J. Hemley, I? M. Bell, and H. K. Mao. Science. 237,605, 1987.

3

4

5

6

7

G . Turrell. InzaredandRaman Spectra of Crystah. Academic, London, 1972. One of the best available texts describing the principles of Raman scatteringfrom crystals. Includes fictor group calculations, polarization measurements, force constant calculations, and many other aspects of crystal physics. B. 0. Mysen, D. Virgo, and E A. Seifert. Rev. Geopbys. Space Pbys. 20,

353,1982. I? Dhamelincourt, E Wallart, M. Ledercq, A. T. N’Guyen, and D. 0. Landon. Ana&. Cbem. 51,414A, 1979.

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J. D. Pasteris, B. Wopenka, and J. C. Seitz. Geochim. Cornrochim.Acta. 52, 979,1988. 9 R.H. Dauskardt, D. K. Vein, and R 0.Ritchie.]. Amur. Cemm. SOC. 72, 1124,1989. i o D. S. Knight and W. B. White. /. Mat. Res. 4,385, 1989. 8

8.2

Raman Spectroscopy

441

8.3

HREELS High-ResolutionElectron Energy Loss Spectroscopy B R U C E E . KOEL

Contents

Introduction Basic Principles Instrumentation Interpretation of Vibrational Spectra Comparison to Other Techniques Conclusions

Introduction

High-resolution electron energy loss spectroscopy (HREELS) has emerged over the past decade as a sensitive, versatile, nondestructive surface analysis technique used to study the vibrations of atoms and molecules in and on solid su&ces.14 Energy loss peaks in the spectra correspond to vibrations of atoms in the surface layers of a solid or in adsorbed molecules. The principal analytical use is to detect the presence of molecular groups on surfaces, often submonolayer amounts of adsorbed species, by observation of their vibrational spectra, and to chemically identify what these species are from the details (positionsand intensities) of the vibrational peaks. For molecular materials, such as polymers, such information may be obtained also about the surface of the solid itself. The information obtained is of the same kind as that in infrared and Raman spectroscopy. The main reasons for the popularity of HREELS over these optical methods are its very small detection limit and its wide spectral range at high surface sensitivity. Most HREELS studies are of single-crystal surfgces, however polycrystalline and noncrystalline samples also can be studied with this technique, but with reduced effectiveness. 442


Chapter 8

Fundamental information from vibrational spectra is important for understanding a wide range of chemical and physical properties of surfaces, e.g., chemical reactivity and forces involved in the atomic rearrangement (relaxation and reconstruction) of solid surfaces. Practical applicationsof H E E L S indude studies OE

2

Functional groups on polymer and polymer film su& Phonon modes of metals and films, semiconductors, and insulators

3

Concentrations of free charge carriers in semiconductors

4

Adsorbed species (and even underlayer atoms) on singlecrystalsurfices

5

Kinetics ofsurfice processes when used in a time-resolved mode.

1

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature ofthe surface chemical bond, molecular structural determination and geometrical orientation, evidence for s&-site specificity, and lateral (adsorbateadsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fibrication have been studied by this technique. The first vibrational spectrum of adsorbed molecules obtained by inelastic scattering of low energy electrons was obtained by Propst and Piper in 1967. Over the next ten years, Ibach in Jiilich achieved much higher resolution and, alongwith several other research groups, developed and used the new technique (HREELS) to study a wide range of surfice vibrations, including polyatomic molecules adsorbed on surfices. The enthusiasm existed mainly because HREELS could be used to study adsorbates on low su&e-area, opaque, metal single-crystal samples, something that could not be done with infrared and Raman spectroscopy. It is now wellestablished as an important vibrational spectroscopyat surfaces, and its utility as an analytical tool with extreme surface sensitivity is rapidly being extended. Basic Principles Electron Scattering

At sufficientlyhigh resolution, quasi-elastically scattered electrons have an inelastic scattering distribution from exciting surfice vibrational modes such as surfice phonons and adsorbate vibrations. (See, for example, Figure 1, the case of CO adsorbed on a Rh surface.) These modes have excitation energies below 0.5 eV (4000cm-l). The basis of the kinematic (single scattering) description of electron scattering from surfices is conservation of energy and momentum parallel to the surface. These conservation laws define the scattering possibilities and, along with the vibrational energies, determinethe peak energies in HREELS. The mechanisms 8.3

HREELS

443

I

I

I

I

I

I

I

2E,=%-AE

- Rhodium

0

Figure 1

500 1000 1500 ENERGY LOSS (on-')

2(

Schematic of electron energy-loss scattering process for electrons of energy Ei striking a Rh single-crystal surface with adsorbed CO molecules present. The actual energy-loss spectrum, due to excitation of CO vibrations, is shown also.

of electron inelastic scattering determine the scattering cross sections (probabilities), and therefore the intensities of vibrational energy loss peaks. Two different electron inelastic scattering mechanisms are most important for explaining the intensities and angular distribution of peaks observed in HREELS: dipole and impact scattering. Dipole scattering is due to the long-range part of the electrostaticinteraction between an incoming electron and the dipolar electric field of the vibrating (oscillating) atoms at the surface. Energy exchange takes place at a long distance (50A) from the surface. This leads to small angle scattering and so the scattered intensity is strongly peaked near the specular direction, with a half-angle A0 0.1-1 '. The dominance of this mechanism for detection at the specular direction leads to the dipole selection rule in HREELS, that is identical to that associated with reflection infrared spectroscopy: Only vibrational modes which produce a

-

444


Chapter 8

dynamic (oscillating) dipole moment perpendicular to the metal surface are dipoleactive, i.e., will produce energy loss peaks on-specular. The origin of this selection rule is the screening of a charge on a conducting surface by an image charge induced in the free electrons. Dynamic dipoles parallel to the surface generate no long-range dipole field, while dynamic dipoles perpendicular to the surface generate a longrange dipole field that is enhanced by a factor of 2. At large scattering angles away from the specular direction, one enters the impact scattering regime. This mechanism is due to a short range electrostatic interaction between an incoming electron and the ion cores of the adsorbate and substrate lattice. Impact scattering is usually several orders of magnitude weaker than dipole scattering at the specular direction in the low-energy regime (below 10 ev). Broad angular scattering distributions having intensities proportional to vibrational amplitudes are characteristic. Selection rules are much less restrictive and are based on adsorbate site symmetry and vibrational mode polarization in relation to the scattering plane of incidence. The impact scattering regime extends to several hundred eV, and the theory of these interactions must include multiple scattering. Measurements of inelastic scattering cross sections in this regime have a great potential for structural analysis, but have been made experimentally only recently. A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on su&es. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in physisorbed layers, with the effects usually observed as small deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. While the underlying mechanisms of HREELS are pretty well understood, many important details relating to selection rules and scattering cross sections remain unknown. Vibrationsat Surfaces

Vibrations give molecular information by identifylng which atoms are chemically bonded together. The frequency of a vibrational mode is related to the bond force constant and reduced mass of the vibrating atoms. Reasonable correlations exist between the number of bonds, bond energy, and force constant for vibrations within molecular species at surfaces and for adsorbat-ubstrate vibrations. It would be extremely useful if one could determine chemical bond strengths from vibrational spectra, but the accurate determination of this quantity is quite ambiguous. Frequency shifts occur as the concentration is changed in adsorbed layers as a result of local bonding variations due to changing sites or bond energies, dipole dipole coupling leading to collective vibrations of the overlayer, or other factors. 8.3

HREELS

445

The intensity of a vibrational mode in H E E L S on-specular is given by the ratio of the inelastic to elastic intensities S = ~ ~ C E ~ ~ ~e) N arccose P ~ F ( ~ ,

(1)

where Sis the dipole scatteringcross section, Eiis the primary beam energy, Nis the number of surface oscillators, Pp is the perpendicular component of the dynamic dipole moment, and f l a y e) is an instrument and geometry factor. This intensity (sensitivity)can be optimized by considering the detected inelastic current, I: (2)

I = IoR(Ei)KS

where 10 is the incident beam current, R(EJ is the metal surfice reflectivity (typically 1-10%) for the primary beam energy E;, and Kis the analyz,er transmission constant. The hnction Sis a smoothly decreasing function of energy and is proportional to E;' for a layer of adsorbed dipoles, but R exhibits significant variation as a function of E+Operationally, the incident energy is tuned to give an intense elastic peak at a low energy (< 10 ev) where Sis large. The sensitivity of H E E L S is largest for molecules, materials, or particular vibrations, that have large S. Normalizingfor the other variables, such as concentration, leads to a general rule that vibrational modes that are observed as strong bands in infrared spectroscopyof gases or of condensed phases will give rise to intense loss peaks in HEELS. For example, carbonyl (C = 0)stretching modes have larger intensities than hydrocarbon C-H stretching modes in IR of bulk phases and also in HREELS on surfaces. Orientational effects will affect this sensitivity in accord with the dipole selection rule. Quantitative analysis and determination of the concentration of surface species requires measuring peak intensities and accounting for vibrational cross sections. This is a difficult task. Careful analysis of Kand analyzer angular characteristics is required when determining S(EJ. To obtain information on the charges on atoms at the surface, one could perform a calculation of dynamic dipole moments (or effective charges) from knowledge of S, in principle. In practice, one usually assumes a point dipole model and gas phase polarizabilitiesfor surface species, and this has lead to anomalouslylow values in the case of adsorbed molecules on metals. Chemical bonding effects greatly enhance the electronic polarizability of chemisorbed molecules and enhance the dielectric screening by the adlayer, reducing the predicted vibrational (inelastic peak) intensity. The width and shape of the energy loss peaks in HREELS are usually completely determined by the relatively poor instrumental resolution. This means that no information can be obtained from HREELS about such interesting chemical physics questions as vibrational energy transfer, since the influence of the time scale and mechanism of vibrational excitations at surfaces on the lifetimes, and therefore the line widths and shapes, is swamped. (Adsorbates on surfaces have intrinsic vibra446


Chapter 8

-

SHIELDING

MONO? HROYATOR

ANALYZER

ELECTRON MULTIPLIER

I

I

L

L

Figure 2

Schematic of a 127" high-resolution electron energy-loss spectrometer mounted on an 8-in flange for studies of vibrations at surfaces.

tional line widths of typically 5 cm-' (0.6 mev) and Lorentzian line shapes.) A practical matter is that this poor resolution is insufficient to resolve closely overlapping vibrational frequencies. Instrumentation

In HREELS, a monoenergetic beam of low energy electrons is focused onto the sample surface and the scattered electrons are analyzed with high resolution of the scatteringenergy (e 10 meV or 80 cm-') and angle (A0 = 2-5"). This is achieved by using electrostatic spectrometers, typically with 127" cylindrical dispersive elements. Hemispherical and cylindrical mirror analyzers have been used also. Some typical analyzer parameters are 25-mm mean radius, 0.1-mm slit width, and 0.5-eV pass energies. Refinements also include the addition of tandem cylindrical sectors to the monochromator and analyzer. A number of commercial versions of spectrometers are capable of routine and dependable operation. A simple spectrometerthat we have used successfully is shown in Figure 2. Electrons from an electron microscope hairpin tungsten filament are focused with a n Einzel lens onto the monochromator entrance slit, pass through the monochromator and exit slit, and are focused on the sample's surface by additional electrostatic 8.3

HREELS

447

lenses (in this case a double plate lens system). The incident beam energy is usually below 10 eV, where dipole scattering cross sections are strong, and the beam current to the sample is typically 0.1-1 nA. Electrons that are reflected from the sample's surfice are focused on the analyzer entrance slit and energy analyzed to produce an electron energy loss (vibrational) spectrum. An electron multiplier and pulse counting electronics are used due to the small signals. Count rates in HREELS are typically 104-106 counts/sec fbr elastically scattered electrons and 10-103 counts/sec for inelastically scattered electrons. Scan times typically range from about five minutes to several hours. The exact incident and scattering angles (+60"from the surfgcenormal) are not critical, but a specular scattering geometry must be attainable. It is also very useM to be able to observe a nonspecular scattering angle either by rotating the crystal about an axis perpendicular to the scatteringplane or by rotating one of the analyzers in the scattering plane. Magnetic shielding must surround the spectrometer because the magnetic field of the earth and any nearby ion pumps will distort the trajectories of the electrons within the spectrometer, because of their small kinetic energies. The spectrometer is mounted in a ultrahigh-vacuum chamber and analysis must be carried out at pressures below lo4 torr. This requirement exists because of the sensitivity of the electron filament and slits to reactions with background gases. Higher pressure gas phase molecules also will cause inelastic scattering that obscures the surface spectra. The applicability of the HREELS technique can be greatly extended by combining it with a high-pressure reaction chamber and sample transfer mechanism. We have previously used this type of system to study hydrogen transfer in adsorbed hydrocarbon monolayers at atmosphericpressure. Interpretationof Vibrational Spectra

In the following discussion, heavy emphasis is made of examples from studies of adsorbed layers on metal single-crystal samples. These illustrate the power of the HREELS technique and represent the main use of HREELS historically. Certainly HREELS has been used outside of the single-crystal world, and mention is made concerning its use on "practical" materials. This latter use of HREELS represents a true frontier. IdentXcation of Adsorbed Species

Determination of surfgce functional groups, e.g., -OH, -C C-, and >C= 0,and identification of adsorbed molecules comes principally from comparison with vibrational spectra (infrared and Raman) of known molecules and compounds. Quick qualitative analysis is possible, e.g., stretching modes involving H appear for v(C-H) at 3000 cm-' and for v(0-H) at 3400 an-'. In addition, the vibrational energy indicates the chemical state of the atoms involved, e.g., v(C=C) 1500an-' and v(C=0) 1800 cm-'. Further details concerning the structure of adsorbates

-

+

448


Chapter 8

Mode assignment

CH@+@3,

CH3C-Rh (111)

v,(CH~)/V,(CD~)

2930 (m)/2192(w) e

2920 (vw)/2178 (vw)e

vS(CH3)/vs(CD3)

2888 (m)/- al

2880 (w)/2065(vw)a]

1420 (m)/1031 (w) e

1420 (vw)/- e

Gs(CH3)/Gs(CD,)

1356 (m)/1002 (vw)a1

1337 (s)/988 (w)al

VKC)

1163 (m)/1882(ms)a1

1121 (m)/1145(m)al

p(CH3)/P (CD3)

1004 (s)/828(s) e

972 (w)/769 (vw)e

vS(M-C)

401 (mV393(m)a1

435 (w)/419 (w)a1

G,(CH3)/Sa(CD3)

* Inremiria of the spectral bands are given in parenthesesfollowing the band frequencies using the following abbreviations: vs = very strong, s = strong, ms = medium mong, m = medium, w = weak, and vw = very w& Symmetry assignments hr each of rhe vibrational modes are also indicated after &e band ikquencies. Table 1

Comparison of the vibrational frequenciee (an-') of the ethylidyne surface species formed on Rh (111)with those of the ethylidyne cluster compound.

comes from comparison to vibrational spectra of ligands in metal cluster compounds whose X-ray crystal structure is known. Isotopic substitution is extremely important in confirming vibrational assignments. Only H / D substitution can be carried out, due to the low resolution, but this is useful for an enormous range of adsorbed molecules, including hydrocarbons. Substituting D for H causes an isotofor those modes wirh largeamplitude H motion. pic shift of As an example, Figure 3a shows the H E E L S spectra after the adsorption of ethylene (H2C = CH2) on Rh(ll1) at 310 IC5 Comparison with gas phase ethylene inhred spectra shows that large changes occurred during adsorption, e.g., v(C-H) = 2880 cm-', indicative of aliphatic C-GH bonds, rather than the expected v(C-H) * 3000 cm-1 for olefinic C=C-H bonds. The complete agreement with frequencies, intensities, and H / D shifts observed in IR spectra (and normal mode analysis) of an organometallic complex, CH~CCO(CO),,~ allowed for the detailed assignment of the loss peaks to vibrational modes of a surface ethylidyne (CCH3) species, as shown in Figure 3b and Table 1. The lack of a welldefined s ular direction for polycrystalline metal samples decreases the signal levels by 10y103, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven use!il for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously fbr a number of metal oxides and compounds, and also semicon-

h

8.3

HREELS

449

a XI183

(CH3 1387

R h (I1 I1 310 K

h

0

1ooo

m o

3000

ENERGY LOSS (cm-')

RhOll) + ethylidyne

Figure 3

450

(a) Specular spectra in HREELS obtsinedfollowing exposure of ethylene or C2D4)on Rh(l11) a t 310 K to form the ethylidyne (CCH,) surface speck.* Ib) The atomic structure (bond distances and angles) of ethyfidyne as deaermined by LEED crystallography.


...

Chapter 8

ductors like Si, InP, and diamond. Dubois, Hansma, and Somorjai fabricated model supported metal catalysts by evaporating rhodium onto an oxidized aluminum substrate and studied CO adsorption by using HREELS. We also have used HREELS to characterize lubricating carbon films on small samples of actual magnetic recording disk heads. Determination of Adsorption Geometry

The symmetry of an adsorbed molecule and its orientation relative to the surface plane can be established using group theory and the dipole selection rule for specular scattering. The angular variations of loss intensities determines the number and frequenciesof the dipole active modes. Only those modes that belong to the totally symmetric representations of the point group which describes the symmetry of the adsorbed complex will be observed as fimdamentals on-specular. The symmetry of the adsorbate-surface complex is then determined by comparing the intensity, number, and frequency of dipole-active modes with the correlation table of the point group of the gas phase molecule. In the previous example of adsorbed ethylidyne, observation of an intense symmetric C-H bending (6,CH3) loss peak and weak antisymmetric C-H bending (6,CH3) loss peak establishes C, symmetry for the surface complex. The adsorption of nitrogen dioxide ( N O 3 on metal surfaces is a beautifid example of linkage isomerism, as illustrated in Figure 4, that was discovered by using HREELS.' Gas phase NO2 has C2v symmetry which is retained in the top two binding geometries (Figures 4b and c) since the asymmetric O N 0 stretching (v,J is not observed on-specular. The symmetry is reduced to C, when NO2 is bonded as the bridging isomer (Figure 4a) and V, is dipole-active. Confirmation of these bonding geometries (and the correct assignment of the rwo C2, isomers) comes from comparison with transition metal complexes containing the nitrite (NO23 ligand. Determination of Adsorption Site

While there is a general pattern of decreased metal-atom stretching frequency with increasing coordination of the adsorption site for the same metal-adatom combination, no site assignments can be made simply by observing the vibrational frequency. Surface chemical bonds clearly control the site dependent vibrational shifts of adsorbed species. Detailed studies that often involve impact scattering can determine the adatom adsorption site in some cases. For polyatomic molecules, bonding to one or more metal atoms at the surface can not be distinguishedin general. Good correlation does exist between the G O stretching frequency (VCO)for adsorbed CO and the adsorption site: vco > 2000 cm-' indicates an atop site (bonding to a single metal atom); 1850 cm-' > vco > 2000 cm-' indicates a bridgesite (bonding to two metal atoms); and vco < 1850 an-' indicates a threefold or fourfold bridge sire (bonding in a region between three or four metal atoms). This correlation has 8.3

HREELS

451

L

vs 1180

I

x125

6

m

0N\ 0 0

800

0.75 ML O/Pt(lll)

0

Figure4

1000 Energy loss / cm-l

2000

Specular spectra in HREELS of NO2 adsorbed in three different bonding geometries?

been established by measuring the vibrational spectrum in conjunction with determining the adsorption site for CO by LEED crystallographycalculations, and also by examining the correlations for CO bonded in organometallic clusters. Similar correlations are likely to exist for other diatomic molecules bonded to surfaces, e.g., NO, based on correlations observed in organometallic clusters but this has not been investigated sufficiently. Figure 5 shows the utility of H E E L S in establishing the presence of both bridge-bonded and atop CO chemisorbed on Pt( 111) and two SnPt alloy surfaces, and also serves to emphasize that H E E L S is very useful in studies of metal al10ys.~ The vcopeaks for CO bonded in bridge sites appear at 1865,1790, and 1845 cm-l on the Pt(l1 I), (2 x 2) and f i surfaces, respectively. The vco peaks for CO 452


Chapter 8

1 ' " ' ' ' ' ' '

1000

0

2000

3000

ENERGY LOSS (cm-1) Figure5

HREELS of the saturation coverage of CO on P t ( l l l 1 and the (2 x 2) and x R30" Sn/Pt surface alloys.'

(b 3 )

bonded in atop sites appear at 2105,2090, and 2085 cm-' on the Pt( 11l), (2 x 2) and surfaces, respectively.Also,lower frequencyvp,co peaks accompany each of the vco peaks. As discussed previously, the peak intensities are not necessarily proportional to the concentration ofeach type of CO species and the exact vco fiequency is determined by many k o r s .

h

OthwAppliestiO~

Many other surfaces can be investigated by HEELS. As larger molecule and nonsingle-crystal examples, we briefly describe the use of HREELS in studies of polymer suhces. The usefulness of HRJZEU specifically in polymer surface science 8.3

HREELS

453

q-; A

I -CH

800

1600

,IC00

scissor

1800

Wave Number [cm-'l

Figure 6

Vibrational spectra of polymers. (a) Transmission infrared spectrum of polyethylene; (b)electron-inducedloss spectrum of polyethylene; (c) transmission infrared spectrum of polypropylene.'0

applications has recently been reviewed by Gardella and Piream.' H E E L S is absolutely nondestructive and can be used to obtain information on the chemical composition, morphology, structure, and phonon modes of the solid surfice. Many polymer surfaces have been studied, including simple materials like polyethylene, model compounds like Langmuir-Blodgett layers, and more complex systems like polymer physical mixtures. Figure 6 shows an H E E L S spectrum from polyethylene [CH3-(CH,),-CH,]. Assignment of the energy loss peaks to vibrational modes is done exactly as described for adsorbates in the preceding seaion. One observes a peak in the C-H stretching region near 2950 cm-', along with peaks due to C-C stretching and bending and C-H bending modes in the "fingerprint" region between 700-1500 cm-' from both the -CH3 (which terminate the chains)and -CHz groups in the polymer. Since the CH3/CH2 ratio is vanishingly s d l in the bulk of the polymer, the high intensity of the -CH3 modes indicate 454


Chapter 8

Figure 7

HREELS vibrational spectra of the interface formation between a polyimide film and evaporated aluminum: (a) clean polyimide surface; (b) with 1/10 layer of AI; (c) with1/2 layer of AI.”

that they are located preferentially in the extreme outer layers of the polymer surface. lo HREELS is useful in many interfacial problems requiring monolayer sensitivity. The incipient formation of the interface between a clean cured polyimide film and deposited aluminum has been studied using HREELS,ll as shown in Figure 7. The film was PMDA-ODA [poly-N,N’-bis(phenoxyphenyl)pyromellitimide], shown schematically in Figure 8. At low AI coverage, the v(C=O) peak at 1720 cm-l is affected strongly, which indicates that Al reacts close to the carbonyl site. At higher AI coverage, new peaks at 2950 and 3730 cm-’ appear which are due to aliphatic -CH, and -OH groups on the surface. This is evidence for bond scissions in the polymer skeleton. In general, the main problems with the analysis of bulk polymers has been charging and rough surfaces. The latter characteristic makes the specular direction poorly defined, which causes diffuse and weak electron scattering. Preparation of the polymer as a thin film on a conducting substrate can overcome the charging problem. Even thick samples of insulating polymers can now be studied using a “flood gun” technique. Thiry and his coworkers’2 have shown that charging effects can be over-

-

8.3

HREELS

455

r

1

-'n

L

PMDA

Figure 8

ODA

Structure of PMDA-ODA.

come by using an auxiliary defocused beam of high-energy electronsto give neutralization of even wide-gap insulators, including AlZO3, MgO, SiO2, LiF, and NaC1.

Comparison to Other Techniques

Information on vibrations at surfaces is complementary to that provided on the compositional analysis by AES and SIMS, geometrical structure by LEED, and electronic structure by X P S and UPS. Vibrational spectroscopy is the most powerful method for the identification of molecular groups at surfaces, giving information directly about which atoms are chemically bonded together. These spectra are more directly interpreted to give chemical bonding information and are more sensitive to the chemical state of surface atoms than those in UPS or X P S . For example, the C( 1s) binding energy shift in XPS between C=Oand G O species is 1.5 eV and that between C=Cand C-C species is 0.7 eV, with an instrumental resolution of typically 1 eV. In contrast, the vibrational energy difference between C=O and G O species is 1000 cm-' and that between C=C and G C species is 500 cm-', with an instrumental resolution of typically 60 cm-'. Vibrational spectroscopycan handle the complications introduced by mixtures of many different surface species much better than UPS or XPS. Many other techniques are capable of obtaining vibrational spectra of adsorbed species: infrared transmission-absorption (IR) and infrared reflection-absorption spectroscopy (IRAS),&s enhanced Raman spectroscopy (SERS), inelastic electron tunneling spectroscopy (IETS), neutron inelastic scattering (NIS), photoacoustic spectroscopy (PAS), and atom inelastic scattering (AIS). The analytical characteristicsof these methods have been compared in several reviews previously. The principle reasons for the extensive use of the optical probes, e.g., IR compared to HREELS in very practical nonsingle-crystal work are the higher resolution (0.28 cm-') and the possibility for use at ambient pressures. HREELS could be &ectively used to provide high surfice sensitivity and a much smaller sampling depth (e 2 nm) and wider spectral range (50-4000 cm-')than many of these other methods. 456


...

Chapter 8

HREELS is used extensively in adsorption studies on metal single crystals, since its high sensitivity to small dynamic dipoles, such as those of C-C and C-H stretching modes, and its wide spectral range enable complete vibrational characterization of submonolayer coverages of adsorbed hydrocarbons.l 3 The dipole selection rule constraint in IR, IRAS, and HREELS can be broken in HREELS by performing off-specular scans so that all vibrational modes can be observed. This is important in species identification, and critical in obtaining vibrational frequencies required to generate a molecular force field and in determining adsorption sites. Conclusions

HREELS is one of the most important techniques for probing physical and chemical properties of suhces. The future is bright, with new opportunities arising fiom continued fundamental advances in understanding electron scattering mechanisms and from improved instrumentation, particularly in the more quantitative aspects of the te~hnique.’~ A better understanding of the scattering of electrons fiom surfaces means better structure determination and better probe of electronic properties. Improvements are coming in calculating HREELS cross sections and surface phonon properties and this means a better understanding of lanice dynamics. Extensions ofdielectric theory of HREELS could lead to new applications concerning interface optical phonons and other properties of superlattice interfaces. Novel applications of the HREELS technique include the use of spin-polarization of the incident or analyzed electrons and time-resolved studies on the ms and sub-ms time scale (sometimes coupled with pulsed molecular beams) of dynamical aspects of chemisorption and reaction. Studies of nontraditional surfaces, such as insulators, alloys, glasses, superconductors, model supported metal catalysts, and “technical” surfaces (samples of actual working devices) are currently being expanded. Many of these new studies are made possible through improved instrumentation. While the resolution seems to be limited practically at 10 an-’,higher intensity seems achievable. Advances have been made recently in the monochromator, analyzer, lenses, and signal detection (by using multichannel detection). New configurations, such as that utilized in the dispersion compensation approach, have improved signal levels by factors of 102-103. Related Articles in the Encyclopedia

EELS, IR, FTIR, and b a n Spectroscopy References 1

8.3

H. Ibach and D. L. Mills. Ek-ctron Energy Loss Spectroscopy andSu$ace vibrations. Academic, New York, 1982. An excellent book covering all aspects of the theory and experiment in HREELS. HREELS

457

W. H. Weinberg. In: Metbod OfExperimentafPLysics. 22,23, 1985. Fundamentals of HREELS and comparisons to other vibrational spectroscopies. 3 vibrational Spectroscopy ofMofecufeson Sufaces. T Yates, Jr. and T. E. Madey, eds.) Plenum, New York, 1987. Basic concepts and experimental methods used to measure vibrational spectra of surfice species. Of particular interest is Chapter 6 by N. Avery on HREELS. 4 vibrations at Surfaces. (R Caudano, J. M. Gales, and A. A. Lucas, eds.) Plenum, New York, 1982; vibrations at Sufaces. (C. R Brundle and H. Morawitz, eds.) Elsevier, Amsterdam, 1983; vibrations at Sufaces 1985. (D. k King, N. V. Richardson and S. Holloway, eds.) Elsevier, Amsterdam, 1986; and vibrations at Sufaces 1987. (A M. Bradshaw and H. Conrad, eds.) Elsevier,Amsterdam, 1988. Proceedings of the International Conferences on Vibrations at Surfaces. 5 B. E. Koel, B. E. Bent, and G. A. Somorjai. Suface Sci. 146,211,1984. Hydrogenation and H, D exchange studies of CCH3(a) on Rh (1 1 1) at 1-atm pressure using HREELS in a high-pressure/low pressure system. e I? Skinner, M. W. Howard, I. k Oxton, S. F. A. Kettle, D. B. Powell, and N. Sheppard. /. Cbem. SOC.,Faradhy Trans. 2,1203, 1981. Vibrational spectroscopy (infrared) studies of an organometallic compound containing the ethylidyne ligand. 7 M. E. Bartram and B. E. Koel. J Vac. Sci. Zcbnol. A 6,782, 1988. HREELS studies of nitrogen dioxide adsorbed on metal surfaces. 8 M. T. PafTett, S. C. Gebhard, R. G. Windham, and B. E. Gel. /. PLys. Cbem. 94,6831,1990. Chemisorption studies on well-characterized SnPt alloys. s J. A. Gardella, Jr, and J. J. Pireaux. Anal. Cbem.62,645, 1990. Analysis of polymer surfaces using HREELS. i o J. J. Pireaux, C. Grdgoire, M. Vermeersch, I?A. Thiry, and R. Caudano. Su$ace Sci. 189/190,903, 1987. Surface vibrational and structural properties of polymers by HREELS. 11 J. J. Pireaux, M. Vermeersch, N. Degosserie, C. Grkgoire, Y. Novis, M. ChtGb, and R Caudano. In: Adbesion and Friction. (M. Grunze and H. J. Kreuzer, eds.) Springer-Verlag,Berlin, 1989, p. 53. Metallization of polymers as probed by HREELS. 12 I? A. Thiry, M. Liehr, J. J. Pireaux, and R. Caudano. J Ehctron Spectrosc. Rekat. Pbenom. 39,69,1986. HREELS of insulators. 2

u.

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Chapter 8

13

B. E. Koel. ScanningEkctron Microscopy 1985/N,1421,1985.The use of HREELS to determine molecular structure in adsorbed hydrocarbon monolayers.

14

J. L. Erskine. CRC Crit. Rev.Solid State Mutez Sci. 13,311,1987.Recent review of scattering mechanisms,surfice phonon properties, and improved instrumentation.

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459

8.4

NMR Solid State Nuclear Magnetic Resonance H E L L M U T ECKERT

Contents

Introduction Basic Principles Structural and Chemical Information from Solid State NMR Line Shapes Instrumentation Practical Aspects and Limitations QuantitativeAnalysis Conclusions

Introduction

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identltj, and quantitate crystalline phases in bulk materials and at s&s and interfaces. Whiie NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under study, NMR can also provide structural insights on a mokcukzr level. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly useful in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often

fail. Due to these virtues, solid state NMR is finding increasing use in the structural analysis of polymers, ceramics and glasses, composites, catalysts, and surfaces. 460


Chapter 8

Examples of the unique insights obtained by solid state NMR applications to materials science include: the Si/Al distribution in zeolites,' the hydrogen microstructure in amorphous films of hydrogenated silicon,* and the mechanism for the zeolite-catalyzed oligomerization of 01efins.~ Basic Principles Nuclear Magnetism and Magnetic Resonance

NMR spectroscopy exploits the magnetism of certain nuclear isotopes.u Nuclei with odd mass, odd atomic number, or both possess a permanent magnetic moment, which can be detected by applying an external magnetic field (typical strength in NMR applications: 1-14 Tesla). Quantum mechanics states that the magnetic moments adopt only certain discrete orientations relative to the field's direction. The number of such discrere orientations is 21 +1, where I, the nuclear spin quantum number, is a half-integral or integral constant. For the common case I = Yz, two distinct orientations (states) result, with quantized components of the nuclear spin parallel and antiparallel to the field direction. Since the parallel orientations are energeticallymore hvorable than the antiparallel ones, the populations of both states are unequal. As a consequence, a sample placed in a magnetic field develops a macroscopic magnetization Mo. This magnetization forms the source of the spectroscopicsignal measured. In NMR spectroscopy the precise energy differences between such nuclear magnetic states are of interest. To measure these differences, electromagnetic waves in the radiofrequency region (1-600MHz) are applied, and the frequency at which transitions occur between the states, is measured. At resonance the condition

holds, where w is the frequency of the electromagnetic radiation at which absorption occurs. The strength of the magnetic field present at the nuclei q,, is generally very close to the strength of the externally applied magnetic field 4 but differs slightly from it due to internal fields ktarising from surrounding nuclear magnetic moments and electronic environments. The factor y, the gyromagnetic ratio, is a characteristic constant for the nuclear isotope studied and ranges fiom lo6 to lo8 rad/Tesla-s. Thus, NMR experiments are always element-selective, since at a given field strength each nuclear isotope possesses a unique range of resonance frequencies. Measurement and Observables

Figure 1 shows the detailed steps of the measurement, from the perspective of a coordinate system rotating with the applied radiofrequency 00 = y&. The sample is in the magnetic field, and is placed inside an inductor of a radiofrequency circuit 8.4

NMR

461

equllibriurn

slgnal Induction and decay (-ms)

b

Figure 1

rf applicatlon (ps)

return t o equillbrium 5 times TI

90' f l l p

repeat sequence

90'Wlse

Detection of NMR signals (a), shown in the rotating coordinate system associated with the oscillating magnetic field component B, at the applied radiofrequency cu, at various stages (+t,) of the experiment: to, spin system with magnetization (fat arrow) at equilibrium; t,, irradiation of the B, field orthogonal t o the magnetization direction tips the magnetization; %, the system after a 90" pulse resulting in transverse magnetization M,; off-resonance precession and free induction decay in the signal acquisition period following the pulse; and t,, return t o spin equilibrium after rpinqattice relaxation; timing diagram of the experiment (b), followed by Fourier transformation.

s,

tuned to the resonance frequency of the nucleus under observation. The magnetization present at time is then detected by applying a short, intense (100-1000 W) radiofrequency pulse (typically 1-10 ps) in a direction perpendicular to B, (tl).The oscillating magnetic component of the radiofrequency pulse stimulates transitions between the magnetic states and tips Mo into the plane perpendicular to the direction of the magnetic field (90' pulse, 5).Following this pulse, the magnetization oscillates in this plane at the transition frequency o and also decays in time due to the various internal interactions present (g).It thereby induces an ac voltage signal in a coil, which is amplified, digitized, and acquired over a typical period of several ms (5).Fourier transformation of this free induction decay (FID) signal then results in the NMR spectrum, a plot of absorption intensity versus frequency. The position, width, and shape of the spectral peaks reflect the local fields present at the nuclei due to internal interactions and allow various chemical conclusions. The area under a spectral peak is directly proportional to the number of nuclei contributing to the resonance, and can be used for quantification purposes. 462


Chapter 8

x

Y

Schematic illustration of the influence of chemical shift upon NMR spectra. See text for further explanation.

Figure 2

Since typical NMR signals are quite weak, extensive signal averaging by repetitive scanning is generally necessary. The pulsing rate at which this can occur depends on the time it takes for the spin system to return into its initial state after the 90"pulse, with Mo along the magnetic field direction (t4).This process can generally be described by first-order kinetics. The associated time constant 7-1, the spin-lattice relaxation time, can vary from a few ms to several hours in solids. Structural and Chemical Informationfrom Solid State NMR Line shapes Internal Interactions

What makes NMR so usell for addressing structural questions in solids is the fact that B1,,, and hence the resonance frequency O, are influenced by various types of internal interactions. These are a direct reflection of the local structural and chemical bonding environments of the nuclei studied, and hence are of central chemical interest. Generally, the observed nuclei experience three types of interactions:6 magnetic dipok-dipole interactions with the magnetic moments from other, nearby nuclei; chemical sbzj interactions with the magnetic fields from the electron clouds that surround the nuclei; and (for nuclei with spin > M ) electric qdrupole interactions with electrostatic field gradients generated by the chemical bonding environment. Each of these interactions is characterizedby a few spectroscopicparameters, which are listed in Table 1. Typically, these parameters are extracted from experimental spectra by computer-fitting methods or are measured by seiectiVe averaging techniques. Due to the simultaneous presence of all three interactions, the resulting solid state NMR spectra can be quite complex. Fortunately, however, in many cases one interaction mechanism is dominant, resulting in spectra that yield highly specific information about local symmetry and bonding. In the following, we will discuss an application of the chemical sh& anisotropy. Figure 2 illustrates that the anisotropic interaction between the molecule and the externally applied magnetic field 8.4

NMR

463

Parameters

Interaction

Chemical shifi (isotropic component)

6i,

Chemical shift anisotropy

6w?

Dipoldipole (homonuclear)

M2(horno)

Dipoldipole

M2(hetero)

6Jy3

L

S

d rignilicance

Magic-angle spinning

Chemical bonding coordinationnumber

Line-shape analysis MAS-sidebands

Coordination

Spin-echo NMR

(rnean-squared local field)

symmetry

Internuclear distances, number of surroundingnuclei

Spin-echo double resonance (SEDOR)

(heteronuclear)

N u d a electric quadrupole

NMR measurement

QCC(quadrupde coupling constant),

Line-shape analysis, nutation NMR

Coordination Symmetry

(asymmetry

Fa) Table 1

Interactions in solid state NMR. parameters, their selective measurement and their structural significance.

induces local magnetic field Components 4, I$,and 4 along the x-, y,and zdirections of a molecular axis system. Quite generally, 4 # By zL& The vector s u m of these components produces a resultant qntalong the direction of 4,the a x i s of quantization, and hence affects the resonance condition. As seen in Figure 2, the magnitude ofBmt(and hence the resonance frequency)will depend cruciallyon the orientation (0, @) of this molecular axis system relative to the magnetic field direction. In a polycrystalline or amorphous material, the orientational statistics lead to a distribution of resonance conditions. Generally, we can distinguish three situations, illustrated in Figure 3a-c: The spectrum in Figure 3c is observed for compounds with asymmetric chemical environments. It shows three distinct features, which can be identified with the different Cartesian chemical shift components a,, aV, and 6, in the molecular axis system. Figure 3b corresponds to the case of cylindrical symmetry, where S, = f &,and hence only two distinct line shape components appear. Finally, for c emical environments with spherical Symmetry the chemical shift is the same in all three directions. Accordingly, the solid state NMR spectrum consists of only a single peak (see Figure 3a). The values of 6ii extracted

Y

464


...

Chapter 8

a

Figure 3

b

C

Characteristic solid state NMR line shapes, dominated by the chemical shift anisotropy. The spatial distribution of the chemical shift is assumed to be spherically symmetric (a), axially symmetric (b), and completely asymmetric (c). The toptrace shows theoretical line shapes, while the bottomtrace shows "real" spectra influenced by broadening effects due to dipolet-dipole couplings.

fiom the spectra usually are reported in ppm relative to a standard reference compound. By definition,

An Example: Chemical Shift Anisotropy in Solid Vanadium Compounds

Figure 4 shows representativesolid state 51VNMR spectra of crystalline vanadates. Each model compound typifies a certain local vanadium environment with welt defined symmetry as shown. One can see fiom these representative data that the solid state 51Vchemical shii anisotropies are uniquely well suited for Merentiating between the various site symmetries. V04-3groups with approximate spherical symmetryyield singlepeak spectra, dimeric V2074 groups (which possess a threefold axis and hence cylindrical symmetry) yield spectra resemblingFigure 3b, while the spectra of the completely asymmetricV021202- groups are ofthe kind shown in Figure 3c. Highly diagnostic line shapes are also observed for vanadium in distorted octahedral environments (ZnV,O,) and in square-pyramidalenvironments (v205)-

An Application: 51VNMR of V oxide films on metal oxide supports

Investigationscarried out within the past few years have revealed that multicomponent metal oxide systems may interact at interf$ces by having one component form a two-dimensional metal oxide overlayer on the second metal oxide component. For example, vanadium oxide can be dispersed on Ti02, Zr02, SiO2, Al203, and 8.4

NMR

465

n J i___

o+-

.

v205

0-’ I

I

,

I

I

200

Figure4

I

,

0

I

I

I

-200

I

,

,

I

-400

,

,

I

-600

,

,

I

,

,

I

,

,

, , ,

-800 -1000 -1200 -1400

PPM

Local microstructures and experimental solid state ”V NMR spectra in crystalline vanadium oxide compounds.

other oxide supports by impregnating the latter with a liquid molecular precursor and following with calcination. Many of these systems are potent oxidation catalysts,with significant inherent advantages to bulk V205. To explore a relationship between the catalytic activity and structural properties, extensive solid state 51V NMR studies have been carried out on these phases.* These studies have benefited greatly from the chemical shiftsystematics discussed above. Figure 5 shows experimental spectra of V surface oxide on y . A l 2 0 3 support. In conjunction with the model compound work one can conclude that two distinctly different vanadia species are present at the surfice: At low vanadia contents, a four-coordinated chaintype species dominates, whereas with increasing surfice coverage a new site emerges whose spectroscopicparameters reveal the presence of a distorted octahedral vanadium environment. Similar trends have been seen with other metal oxide supports, 466

VIBRAT:ONAL SPECTROSCOPIES

...

Chapter 8

1.0 (0.05)

200

0

-200

-400

-600

-800 -1000 -1200

ppm

Solid state 51V NMR spectra of Vanadium oxide on ralumina as a function of vanadium loading (wt.%)and surface coverage 0. Notethe gradual emergence of the six-coordinatedvanadium site with increased loading.

Figure 5

although the type of vanadium environment in the overlayer also depends strongly on the acidity of the surface. SelectheAveraging Techniques

In general, the specific information that can be obtained from a simple solid state NMR experiment depends on the “personality”of the nuclear isotope under study. In many cases, solid state NMR spectra are not as straightforwardlyinterpretable as in the preceding example. Furthermore, disordered materials, such as thin films, 8.4

NMR

467

-300 Figure 6

-400

-500

-600

-700

-800

-900 PPM

Solid state 51V static and magic-angle spinning NMR spectra of a-Mg2V20,. This compound has two crystallographically distinct vanadium sites. While the static spectrum is a superposition of two powder patterns of the kind shown in Figure 3, MAS leads to well-resolvedsharp resonances. Weak peaks denoted by asterisks are spinning sidebands due to the quadrupolar interaction.

glasses, and composites, often show only broad and unresolved spectra, because in such samples the spectroscopicparameters are subject to distribution effects. Here, the diagnostic character of solid state NMR can be enhanced dramatically by selective averaging techniques. The idea is to simplify the spectra by suppressing certain interactions while preserving others for analysis. The most popular and most widely applied experiment is to acquire the NMR spectrum while rotating the sample rapidly about an axis inclined by 54.7" (the "magic" angle) relative to the magnetic field direction. This technique, called Magic-Angle Spinning (MAS), results in an average molecular orientation of 0 = 54.7" relative to the magnetic field over the rotation period, regardless of the initial molecular orientation. Theory predicts that at this specific angle the anisotropy of all internal interactions (which scale with the factor 3c0s~e-l) vanishes. Consequently, MAS converts broad powder patterns of the kind shown in Figure 3a-c into highly resolved sharp resonances that can be straightforwardly assigned to individual sites. For example, Figure 6 illustrates the superior ability ofMAS to resolve the crystallographicallydistinct vanadium sites in the model compound a-Mg2V207. The high resolution obtained by MAS and the simplicity of the spectra make solid state NMR a particularly useful technique fbr identifying crystalline phases in the bulk or at surhces and interfaces. A number of other, more sophisticated, selective averaging tools (includingspin echo, double resonance and two-dimensional techniques) are available, both for spectral editing purposes and for obtaining quantitative information about inter468


Chapter 8

atomic However, among all these techniques, the conceptually simple MAS-NMR experiment has had by far the biggest impact in materials science applications. Instrumentation

NMR instrumentation consists of three chief components: a magnet, a spectrometer console, and a probe. While in the past much solid state NMR research was conducted on home-built equipment, the current trend is toward the acquisition of commercial systems. The magnets used for solid state NMR applications generally are superconducting solenoids with a cylindrical bore of 89-mm diameter. The most common field strengths available, 4.7,7.0,9.4, and 11.7 Tesla, correspond to proton resonance frequencies near 200,300,400, and 500 MHz, respectively. The spectrometer console comprises a radiofrequency part for the generation, amplification, mixing, and detection of radiofrequency and NMR signals, and a digital electronicspart, consisting of a pulse programmer, a digitizer, and an on-line computer. Equipment normally used for pulsed liquid state NMR applications often can be modified for solid state experiments by adding high-power amplifiers (up to 1-kW output power) and fast digitizers (2 MHz or faster). NMR probes are used to transfer the radiofrequency pulse to the sample and to detect the nuclear induction signal after the pulse. They contain radiofrequencycircuitry, which is tunable to the nuclear resonance frequency via variable capacitors and which is based usually on a single solenoidal coil (diameter 4-25 mm). MA!3NMR experiments require special probes, enabling fast sample rotation within the magnet. Currently, MAS is done mostly on powdered samples packed within cylindrical containers (rotors) that are machined from single-crystal alumina, zirconia, or silicon nitride to precise dimensions. High-pressured gases (air, Nz, or Ar, at 4060 lb/in2) thrusting on turbine-shaped caps are used to accomplish fast rotation. For routine experimenrs, typical spinning speeds are 5-10 kHz; with suitable equipment up to 20 kHz can be reached. PracticalAspects and Limitations

Sample preparation requirements in solid state NMR are strikingly simple because the measurement is carried out at ambient temperature and pressure. Wide-line NMR experiments can be carried out on solid samples in any form, as far as the sample dimensions fit those of the coil in the NMR probe. MA!3 experiments require the material to be uniformly distributed within the rotor. Compared to other spectroscopicmethods, NMR spectroscopyis a very insensitive technique. As a general rule of thumb, the sample studied must contain at least 10-5 moles of target nuclei. The required sample size thus depends on the percentage of the element present in the sample, as well as on the natural abundance of the 8.4

NMR

469

NMR isotope measured. For example, for the detection of phosphorus by 31P NMR in a sample containing 3 wt.% phosphorus, approximately 10 mg of sample are required. By contrast, the corresponding detection limit for 29Siin a similar situation is 22 times higher, due to the much lower natural abundance (4.7%)of the "Si isotope. Naturally, the low sensitivityposes a particular obstacle to NMR studies of thin films and surfaces. Large surface areas are obviously favorable (the samples in Figure 5 have surface areas around 150 m2/g), but good results can often be obtained on samples with surfice areas as small as 10 m2/g. Experimentally, the detection sensitivity can be increased by increasing the applied field strength; by increasing the sample size (although practical considerations often impose a maximum sample volume of several cm3); and by using special NMR techniques (crosspolarizationP6) for sensitivityenhancement. Additional limitations arise from the nuclear electric quadrupole interaction for nuclei with I > M and fiom the dipolar interaction of nuclei with localized electron spins in paramagnetic samples. Both interactions tend to interfere with the alignment of the nuclear spins in the external magnetic field, and to make the observation of NMR signals difficult. Due to these factors, less than half the elements in the periodic table are conducive to solid state NMR experiments. The following ranking holds with regard to detection sensitivityand general suitability in the solid state-highly favorable elements: H, Li, Be, B, F, Na, Al, P,V, Sn, Xe, Cs, Pt, and T1; less well-suited elements, where NMR often suffers from sensitivityrestrictions: C, N, Si, Se, Y, Rh, Ag,Cd, Te, W, Hg, and Pb; and elements whose suitability is often limited by quadrupolar interactions: N, 0,C1, Mn, Co, Cu, Ga, K, Rb, Nb, Mo, In, and Re. Elements not listed here can be considered generally unsuitable for solid state NMR.

Quantitative Analysis

In contrast to other spectroscopies, such as IWRarnan or VIS/W, NMR spectroscopy is inherently quantitative. This means that for a given nucleus the proportionality factor relating the area of a signal to the number ofnuclei giving rise to the signal is not at all sample-dependent. For this reason, NMR spectroscopy has been used extensively for absolute and relative quantitation experiments, using chemically well-defined model compounds as standards. It is essential, however, to follow a rigorous experimentalprotocol for such applications. To maintain the quantitative character of NMR spectroscopy, the repetition rate of signal averaging experiments has to be at least five times the longest spin-lattice relaxation time present in the sample. This waiting period is necessary to ensure that the magnetization is probed in a reproducible state, corresponding to thermodynamic equilibrium. 470


Chapter 8

Conclusions To date, the simple one-pulse acquisition experiments (with or without h4AS) reviewed here have been the mainstay for the majority of NMR applications in materials science. A current trend is the increasing use of NMR for in situ studies, using more sophisticated hardware arm~gements.~~ For the near future, a rapid diffusion of NMR know-how and methodology into many areas of solid state science can be foreseen, leading to the application of more complicated techniques that possess inherently greater infbrmational content than MAS-NMR Examples of this kind include multiple pulse techniques, such as one- and two-dimensional versions of spin-echo and double resonance methods, and experiments involving variable rotation angles? Also, new areas for applications are opening up. A most recent development has been the successful demonstration of three-dimensional imaging of ceramic and polymeric m t d s by solid state NMR techniques. This area is most likely to expand considerably. Related Articles in the Enqdopedia

EXAFS,FTIR, XRD References 1

2

3

4

J. Klinomki. h g . NMRSpectrwsc. 16,237,1984. A summary of 23Si MAS-NMR applicationsto zeolites. J. Baum, K. K. Gleason,A. Pines, A. N. Garroway, and J. A. Reimer. Pbys, Rev- Lett. 56,1377,1986. Detection ofhydrogen clusteringin amorphous hydrogenated silicon by a special technique of dipolar spectroscopy, multiple-quantum NMR J. E Haw, B. R. Richardson, LS. Oshiro, N.D. Lam, and J. A. Speed. J Am. Cbem. SOC.111,2052, 1989. In situ NMR studies of catalyticproperties. T . M. Duncan and C. R. Dybowski. Su$Sci. Rep. 1,157,1981. An excellent review of relevant NMR theory, modern techniques, and applications to surfices.

5

B. C. Gerstein and C. R. Dybowski. Transient Ecbniques in NMR of Sol-

id.Academic Press, 1985. An in-depth treatment of the theoretical foun6

0.4

dations of solid state NMR M. Mehring. Principles ofHigh Resolution NMR in Solia?. Springer Verlag, New York, 1983. An in-depth treatment ofthe theoretical foundations of solid state NMR NMR

47 1

7

B

9

H. kkert. Bet: Bunsenges. Pbys. G e m . 94,1062,1990. Arecent review of modern NMR techniques as applied to various Materials Science problems. H. Eckert and I. E. Wachs./. Pbys. Cbm. 93,6796, 1989. 51VNMR studies of vanadia-based catalysts and model compounds. J. E Stebbins and I. Farnan. Science. 245,257, 1989. Highlights in situ NMR applications at ultrahigh temperatures.

472


Chapter 8

9 I O N SCATTERING TECHNIQUES Rutherford Backscattering Spectrometry, RBS 476 Elastic Recoil Spectrometry, ERS 488 9.3 Medium-Energy Ion Scattering Spectrometrywith Channeling and Blocking, MEISS 502 9.4 Ion Scattering Spectroscopy, ISS 514 9.1 9.2

9.0

INTROD UCTlO N

In this chapter three ion-scattering methods for determining composition and geometric structure (for single crystal material) are discussed. They are Rutherford Backscattering Spectrometry, RBS, which typically utilizes high-energy He or H ions (usually 1-3.4 MeV energies), Medium-Energy Ion Scattering, MEIS (ion energies from 50 keV to 400kev), and low-energy ion scattering (100eV to 5 kev) which is more commonly known as Ion-Scattering Spectroscopy, ISS. A fourth technique, Elastic Recoil Spectrometry, ERS, is an auxiliary to these methods for the specific detection of hydrogen. All the techniques are performed in vacuum. For the three ion-scattering techniques there are differences in information content that are a consequence of the different ion energy regimes involved, plus some differences in instrumentation. For RBS, the most widely used method, the highenergy ions penetrate well into the sample (up to 2 pn for He ions; 20 pm for H ions). On its way into the sample an individual ion loses energy in a continuous manner through a series of electronic scatteringevents. Occasionally an ion undergoes a billard ball-like collision with the nucleus of an atom in the sample material and is back scattered with a discrete, large energy loss, the value of which is characteristic of the atom struck (momentum transfer). Since this major energy loss is atom specific, whereas the small continuum energy loses depend on the depth traveled, the overall energy spectrum of the emerging back scattered ions reveals both the elemental composition and the depth distribution of those elements in a nondestructive manner. Since the scattering physics is quantitatively well understood at 473

these high energies (Rutherford Scattering) a standardless depth profile is obtainable with a few percent accuracy. Other important factors are: the separation in backscattering energy of adjacent elements in the backscattered spectrum decreases with increasing mass such that Ni and Fe are not separable, whereas C and 0 are easily distinguished; the backscattering cross section is essentially proportional to Z2and therefore heavy elements in light matrices have much better detection limits (by about a factor of 100) at 10-100 ppm than vice versa; the depth-resolution depends on ion energy, angle of incidence, and depth below the surhce such that a resolution of 20 is achievable (low ion energy, grazing angle, analysis done right at the surface), but more typical values are several hundred angstroms. For single crystal materials, aligning the ion beam with a crystallographicdirection suppresses the signal from below the first few layers, since the atoms in these layers shadow bulk atoms below from the incoming ion beam. This technique, known as channeling, is used both to enhance the surface sensitivity and to determine the extent of crystallinedefects, since if atoms are displaced from their correct positions the degree of shadowing in the channeling mode will be decreased. MEIS is a more sophisticated form of RBS that uses lower energy ions (usually 1 0 0 4 0 0 kev) and a higher resolution ion energy analyzer. The lower energies restrict the probing depth. The better energy resolution improves the depth resolution down to a few angstroms. It also improves the ability to distinguish elements at high mass. When used for single crystal materials in conjunction with channelling of the incoming ions, and blocking of the outgoing backscattered ions, the method provides atomic positions at a surface, or an interface up to 4 or 5 layers below the surface, to an accuracy of a few hundredths of an angstrom. In addition it retains the standardlessquantitation of the RBS method with sensitivitiesto submonolayer amounts. Both RBS and MEIS are extremely expensive, requiring an ion accelerator. The lower energy accelerator of MEIS is cheaper, but this is counteracted by the greater expense of the more sophisticated ion energy analysis. Both techniques typically cost around $1,000,000 and take up large laboratories. Beam diameters are usually millimeters in size, but microbeam systems with spatial resolution down to 1 jun exist. Ion-beam damage can be a problem, particularly for polymers. It can be mitigated by using low ion doses and by rastehng the beam. ISS involves the use of ions (usually He or Ar) in the 100-5000 eV range. At these energies essentially only backscattering from atoms in the outermost atomic layer produces peaks in the ion energy spectrum due to nearly complete neutralization of any ions scattered from below the surfice. As with RBS and MEIS the ability to resolve adjacent elements becomes rapidly poorer with increasing Z This can be mitigated, but not solved entirely, by changing the mass of the ion (eg Ar for He), the ion energy, ~d the angle of detection. All these variations significantly affect the scattering cross section and background, however, which complicates quantitative use. Quantitation is not standardlessat these energies but requires suitable standards to determine relative cross sections for the set of scattering parame474

ION SCATERING TECHNIQUES

Chapter 9

ters used. Cross sections still depend roughly on 2: however, so the technique is much more sensitive to high-2 materials. Owing to its extreme surface sensitivity ISS is usually used in conjunction with sputter profiling over the top 50 A or so. Spatial resolution down to about 150 pm is routinely obtained. The technique is not widely used owing to the lack of commercial equipment and its poor elemental resolution. Instrumentation is quite cheap, and simple, however, since an ordinary ion gun replaces the ion accelerator used in RBS and MEIS. It can be used as an auxiliary technique on X P S or AES spectrometers by reversing the voltage on the analyzer to pass ions instead of electrons. In ERS, also known as Forward Recoil Spectrometry, FRS, Hydrogen Recoil Spectrometry, HRS, or Hydrogen Forward Scattering, HFS, hydrogen atoms present in a sample recoil from He ions striking the sample at grazing angle with sufficient forward momentum to be ejected. They are then separated from any He that also emerges by using a thin stopping foil that allows energetic H to pass but not He. In this way the hydrogen content can be quantitatively determined. The technique can be applied in RBS, MEIS, or ISS spectrometors and is used because a target atom that is lighter than the incident ion is only scattered in the forward direction; it is never backscattered. Therefore regular RBS cannot be used for H detection. The depths analyzed and depth-profiling capabilities are similar to those of the equivalent backscattering methods, but the depth resolution is poor (2500 A at 1000-8, depths). NRA (Chapter l l ) , an alternative technique for detecting hydrogen, has greater sensitivity than ERS. SIMS (Chapter 10) has far greater sensitivity for hydrogen (down to trace amounts) than either technique and better depth resolution, but it is a destructive sputter-removal method and is difficult to quantify. Sample damage can also be a problem with ERS, particularly for polymers.

475

9.1 Rutherford Backscattering Spectrometry SCOTT M. B A U M A N N

Contents

Introduction Basic Principles Channeling Quantification Artifacts Instrumentation Applications Conclusions

Introduction

Rutherford Backscattering Spectrometry (RBS) is one of the more quantitative depth-profiling techniques available, with typical accuracies of a few percent. The depth profiling is done in a nondestructivemanner, i.e., not by sputtering away the surface layers. Results obtained by RBS are insensitive to sample matrix and typically do not require the use of standards, which makes RBS the analysis of choice for depth profiling of major constituents in thin films. Detection limits range from a few parts per million (ppm) for heavy elements to a few percent for llght elements. RBS depth resolution is on the order of 20-30 nm, but can be as low as 2-3 n m near the surface of a sample. Typical analysis depths are less than 2000 nm, but the use of protons, rather than helium, as the probe particle can increase the sampling depth by as much as an order of magnitude. Lateral resolution for most instruments is on the order of 1-2 millimeters, but some microbeam systems have a resolution on the order of 1-10 pm. 476

ION SCAlTERING TECHNIQUES

Chapter 9

Three common uses of RBS analysis exist: quantitative depth profiling, areal concentration measurements (atoms/an2), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profiling of polymersY1 high-Tc superconductors,optical coatings, and catalyst particles2 Recent advances in accelerator technology have reduced the cost and size of an RBS instrument to equal to or less than many other analytical instruments, and the development ofdedicated RBS systems has resulted in increasing applicationof the technique, especially in industry, to areas of materials science, chemistry, geology, and biology, and also in the realm ofparticle physics. However, due to its historical segregation into physics rather than analytical chemistry, RBS still is not as readily available as some other techniques and is often overlooked as an analytical tool. Basic Principles

RBS is based on collisions between atomic nuclei and derives its name from Lord Ernest Rutherford who first presented the concept of atoms having nuclei. When a sample is bombarded with a beam ofhigh-energy particles, the vast majority of particles are implanted into the material and do not escape. This is because the diameter of an atomic nucleus is on the order of 1O4 while the spacing between nuclei is on the order of 1 k A small fraction of the incident particles do undergo a direct collision with a nucleus of one of the atoms in the upper fav pm of the sample. This “collision” actually is due to the Coulombic force present between two nuclei in close proximity to each other, but can be modeled as an elastic collision using classical physics. The energy of a backscattered particle detected at a given angle depends upon two processes: the loss of energy by the particle due to the transkr ofmomentum to the target atom during the backscatteringevent, and the loss of energy by the particIe during transmission through the sample material (both before and after scattering). Figure 1 is a schematic showing backscattering events occurring at the surface of a sample and at a given depth din the sample. For scattering at the sample’s surface the only energy loss is due to momentum transfer to the target atom. The ratio of the projectile’s energy after a collision to the its energy before a collision (E,/&) is d&ned as the kinematic factor IC3,

a

*

where MI is the mass of the incident particle (typically*He); M, is the mass of the target atom; and R is defined as the angle between the trajectory of the He particle before and after scattering. 9.1

RES

477

0

INCIDENT PARTICLES M 1 M2

0 ATOMS N TAR.mmHNERGYLoss

--1 Figure 1

I

A schematic showing the various energy IOU processes for backscattering from a given depth in a sample. Energy is lost by momentum transfer betweenthe probe particle and the target particle, and as the probing particle traverses the sample material both before and after scattering.

As shown in Figure 1, when the probing particles penetrate to some depth in a sample, energy is lost in glancing collisions with the nuclei of the target atoms as well as in interactions with electrons. For a 2-MeV He atom, the energy loss is in the range of 100-800 eV/nm and depends upon the composition and density of the sample. This means that a particle that backscatters from some depth in a sample will have measurably less energy than a particle that backscatters from the same element on the sample's surface. This allows one to use RBS in determining the thickness of layers and in depth profiling. The relative number of particles backscattered from a target atom into a given solid angle for a given number of incident particles is related to the differential scattering cross section: do 1#2 =

Z ~ Z ~ 2C4Z ( J 1 - ( ( M , / M ~sine)2+cose) )

2

(7 (sine)*,/l)( (Ml/M2) sine12

(2)

where 2 1 and 2, are the atomic numbers of the incident atom and the target atom, Eis the energy of the incident atom immediately behre scattering, and cis the electronic charge. A rule of thumb is that the scatteringcross section is basically proportional to the square of the atomic number Zof the target species. This means that RBS is more than a hundred times more sensitive for heavy elements than for light 478

ION SCAITERING TECHNIQUES

Chapter 9

5E4-

incident He12.275 MeV, Detector Angle-1 60' .----230nm TaS12 590nm TaSi.3

-

.;-.-. . . . I

.

a

a-

x 0

0.200

0.600

1.000

1.400

1.800

2. 00

Backscattering Energy (MeV)

Figure 2

RBS spectra from two TaSi,films with different Si /Ta ratios and layer thicknesses.

elements, such as B or C. There is much greater separation between the energies of particles backscattered from light elements than from heavy elements, because a significant amount of momentum is transferred from an incident particle to a light target atom. As the mass of the target atom increases, less momentum is transferred to them and the energy of the backscattered particle asymptotically approaches the incident particle energy (see Equation 1). This means that RBS has good mass resolution fbr light elements, but poor mass resolution for heavy elements. For example, it is possible to resolve C from 0 or P from Si but it is not possible to resolve W from Ta, or Fe from Ni when these elements are present at the same depths in the sample, even though the difference in mass between the elements in each of these pairs is roughly 1 amu. Figure 2 shows how the processes combine to create an RBS spectrum by displaying the spectra from two TaSi, films on Si substrates. Met4 silicide films are commonly used as interconnects between semiconductor devices because they have lower resistivity than aluminum or polysilicon. The resistivity of the fdm depends upon the ratio of Si to metal and on the film thickness, both of which can be determined by RBS. The peak in each spectrum at high energy is due to scattering from Ta in the TaSi, layers while the peak at lower energy is from Si in the TaSi, layer and the Si substrate. The high-energy edge of the T a peaks near 2.1 MeV (labeled A) corresponds to scattering from Ta at the surface of both samples, while the highenergy edge of the Si peaks (labeled 0) near 1.3 MeV corresponds to backscattering from Si at the surfice of the TaSi, layer. By measuring the energy width of the Ta peak or the Si step and dividing by the energy loss of He (the incident particle) per unit depth in a TaSi, matrix, the thickness of the TaSi, layer can be calculated. For example, the low-energy edge of the Ta peak corresponds to scattering from Ta at the TaSi,Si interface and the step in the Si peak corresponds to the increase in the 9.1

RBS

479

ing He will backscatter from the first few monolayers of material at the same rate as a nonaligned sample, but backscattering from buried atoms in the lattice will be drastically reduced, since these atoms are shielded from the incident particles by the atoms in the surface layers. For example, the backscattering signal from a singlecrystal Si sample that is in channeling alignment along the (100) axis will be approximately 3% of the backscattering signal from a nonaligned crystal, or amorphous or polycrystalline Si. By measuring the reduction in backscattering when a sample is channeled it is possible to quantitatively measure and profile the crystal perfection of a sample, or to determine its crystal orientation. Figure 3 shows channeled spectra from a series of Si samples that were implanted with 1013, and 1015arsenic atoms/an2. Only the As peaks for the two highest dose implants are shown, but with a longer data acquisition time the concentration 1013As atoms/cm2 could be detected. The damage caused to the Si crystal lattice by the As implants is reflected in the peaks near 1.25 MeV in the aligned spectra. In the case of the 1015-atoms/cm2implant there is little or no single-crystal structure remaining in the damaged region of the Si, so the backscattering signal is the same height as for nonaligned Si. Measuring the energy width of the damage peak indicates that the damaged layer is approximately 200 nm thick. Integrating the damage peak and subtracting the backscatteringsignal obtained for the nonimplanted reference indicates that approximately 1.0 x 10l8 Si atoms/cm2 were dis laced by the 10'5-atoms/cm2 As implant, while 3.4 x 1017 and 1.7 x 10P6 Si atoms/cm2 were displaced by the 10'4-atoms/cm2 and 1013-atoms/cm2

As implants, respectively. In this case RBS could be used to measure accurately the total concentration of arsenic atoms implanted in each sample, to profile the As implant, to determine the amount ofAs that is substitutional in the Si lattice and its lattice location, to measure the number of displaced Si atoms/cm2, and to profile the damage in the Si crystal.

Quantification

As noted above, the calculation of elemend concentrations and thicknesses by RBS depends upon the scattering cross section of the element of interest and the stopping cross section of the sample matrix. The scattering and stopping cross sections for each element have been carellly measured and In general, scattering cross sections fbllow the Rutherford scattering model to within 5%. It is difficult to accurately describe the stopping cross sections for all elements with a single equation, so semiempiricalvalues are employed. A polynomial equation with several terms is used so that the stopping cross sections for each element can be calculated over a range of energies. In general, the calculated stopping cross sections are accurate to 10Yo or better. The stopping cross section for a multi-elemental sample is calculated by normalizing the stopping cross section of each element to its concentration in the sample. 43

9.1

RBS

'

481

12000

9000

--

6000

-Si Substrate

3000

-

--

0-

3000

--

z

I .

?

UI

. , 0.600

1.doo l.io0 Backscattering Energy (MeV)

I!

i

0

a .+.

d b

Figure 4

30

RBS spectra from a sample consisting of 240 nm of Si on 170 nm of Si02 on a Si sub-ate. The spectrum in (a) was acquired using a scattering angle of leOo while the spectrum in (b) used a detector angle of l l O o . This sample was implanted with 2.50 x 10" As atoms/cm*, but the As peak cannot be positively identifiedfrom either spectrum alone. Only As at a depth of 140 nm will produce the correct peak in both spectra.

Due to the convoluted mass and depth scales present in an RBS spectrum, it may not be possible to accuratelydescribe an unknown sample using a single RBS spectrum. For example, Figure 4a is an RBS spectrum acquired at a backscatteringangle of 160' from a sample implanted with 2.50 x 10l6atoms/cm2 of& at a depth of approximately 140 nm. If this were a totally unknown sample it would not be possible to determine positively the mass and depth of the implanted species fiom t h i s spectrum alone, since the peak in the RBS spectrum also could have been caused by a heavier element at greater depth, such as Sb at 450 nm, or M o at 330 nm, or by a 482

ION SCAlTERlNG TECHNIQUES

Chapter 9

lighter element at a shallower depth, such as Ga at 80 nm. If an additional spectrum is acquired at a glancing backscattering angle, the scattering kinematics will be changed and the backscattered particles will have a longer escape path through the sample material to the detector. As the detector angle approaches 90" (tangent to the sample surface) the backscattering peak for a buried element will be shifted to lower energies due to the greater loss of energy along the longer escape trajectory out of the sample. Figure 4b is the RBS spectrum acquired from the same sample but at a backscattering angle of 110". Shown in this figure are the locations for the other possible elements and depths that would match the peak shown in Figure 4a. Only As at a depth of 140 nm will produce a peak at the correct energy in both spectra. By acquiring two backscattering spectra at different angles it is usually possible to determine the depth and mass of an unknown element. One should note also that the depth resolution for the surface Si layer and the Si02 layer are improved in the 11Oo spectrum due to greater energy loss per unit depth in the sample. This results in the wider peaks for the surface Si and Si02 layers in the 110' spectrum. Artifacts

Although RBS does not suffer from matrix effects that are normally associated with profiling techniques using sputtering, such as SIMS, AES, or SNMS, there are other factors that do limit the application of the technique. The convoluted nature of the mass and depth information available in an RBS spectrum often results in a spectral interference between the peak for a light element and a buried heavier element. For example, in Figure 4a the He that backscatters from the oxygen in the Si02 produces the peak between 0.65-0.72 MeV, while backscattering from Si in the Si02 produces the peak between 1.2-1.3 MeV. Scatteringfrom the Si substrate produces the peak between 0-1.2 MeV (the backscattering signal has been suppressed between 0-0.35 MeV). The peak from the Si substrate contributes noise to the oxygen peak and limits the accuracy to which the oxygen concentration can be measured. In cases where the matrix contains heavy elements it may not be possible to detect light elements at all, i.e., carbon in a bulk tungsten sample. Procedures have been developed to eliminate or minimize the effects of these spectral interferences. These include channeling crystalline substrates to reduce the backscattering signal from the substrate, using detectors at glancing angles to the sample's surface or orienting the sample at a glancing angle to the incident ion beam, and varying the energy of the incident ion. The repeatable nature of RBS allows the use of computer models to predict the RBS spectrum from a given sample structure, permitting the investigator to optimize the measurement parameters or the sample structure to maximize the accuracy and usefulness of the results. Sample roughness also can produce problems in the interpretation of RBS spectra that are similar to problems encountered by sputtering techniques like AES, 9.1

RBS

483

SIMS, and SNMS; in rare cases, such as for HgCd,Tel,samples or some polymers, the sample structure can be modified by the incident ion beam. These effects can often be eliminated or minimized by limiting the total number of particles incident on the sample, increasing the analytical area, or by cooling the sample. Also, if channeling of the ion beam occurs in a crystal sample, this must be included in the data analysis or serious inaccuracies can result. To avoid unwanted channeling, samples are often manipulated during the analysis to present an average or “random” crystal orientation. Finally, the fundamental unit of concentration obtained by RBS is in atoms/cm2 or concentration in the sample-versus-backscattering energy loss. To convert the profile of a backscattering peak into a depth profile it is necessary to assume a density for the material being profiled. For single-element films, such as Si, Ti, and W, an elemental density can be assumed for the film and an accurate thickness is obtained. In the case of multi-elemental films with an unknown density, a density for the film is calculated by summing the density of each element, normalized to its concentration. The accuracy of this assumption is usually within 25%, but for some cases the actual density of the film may vary by as much as 50%100% from the assumed density. It is useful to note that:

TRBsx DRBs= (atoms) /cm 2 = Drealx

qeal

(3)

where T& and hS are the thickness obtained by RBS and the density assumed to calculate this thickness; and T d and Drd are the actual physical thickness and density of the film. If the physical thickness of a film can be measured by some other technique, such as SEM, TEM, or profilometry, then the actual film density can be accurately calculated. Instrumentation

An RBS instrument can be divided into two basic components: the particle accelerator and the analysis chamber or end station. PIXE and ERS analyses employ similar instrumentation, but use different incident ion beams or detectors. Particle Accelerators

Two types of particle accelerators are used to obtain the MeV energies used for RBS. Single-ended accelerators are similar to ion implanters used in the semiconductor industry but have an ion source located at the high-energy terminal of the accelerator. Ions are extracted from the source and are accelerated down the beam line to ground potential. Tandem accelerators use a source that is at ground potential and that emits a beam of negative ions that are accelerated toward the positively charged terminal of the accelerator,where their charge states are changed by passing the beam through a thin foil or a gas cell. The (now) positively charged particles are accelerated to higher energy as they are repelled from the positive terminal voltage and back to ground potential. 484

ION SCATTERING TECHNIQUES

Chapter 9

End Station

A multitude of analysis chambers exist that have been designed with specific measurements or sample sizes in mind. State-of-the-art systems have a multiple-axis goniometer, which allows positioning of many samples for analysis without breaking vacuum. High precision (on the order of &O.Olo) is required when orienting samples for channeling, which often makes the goniometers used for channeling both complicated and expensive. The minimum sample size is controlled by the dimensions of the incident ion beam. Typically, the ion beams used for RBS are about 1-2 mm in diameter and samples are between 0.1-1 cm2 in area, however, some microbeam systems with beam diameters on the order of 10 pm have been built. Analysis chambers also have been made to accommodate large samples, such as entire silicon wafers. For the purposes of most standard RBS measurements the analysis chamber needs to be evacuated to at least torr. Extremely good depth resolutions of less than 3 nm can be obtained by orienting either the incident ion beam or the detector at a glancing angle to the sample surface. Applications

Listed below is a summary of some common applications of RBS. Semiconductors:

Quantitative depth profiling of: Metal silicide films (WSi,, MoSi,TiSi,, etc.) Barrier metals (TIN,, TiW,, etc.) Insulating layers (SiO,, SiN,, and S i 0 2 3 Cu in AI interconnect 111-Vand11-VImaterials(AlxGal&, andHg,Cdi-%Te) Metal multilayer stacks DoseAattice substitutionality of implanted species Crystal damage versus depth (Si, SiGe,, Al,Gal-As, and Hg$dl-xTe)

High-Tc superconductors:

Quantitative depth profiling (Y€3a.$u+3,, and BiSr&u,Ca,OJ Crystal orientation and damage versus depth

Optical/antir&ective Quantitative depth profiling of multilayered stacks coatings: (Si02, HfO2, Ti02, SnO2, InSnxOy,etc.) Polymers:

Depth profiling of halogens and impurities Metallization of surfaces

Catalysts:

Location of active ions on or in partides

9.1

RBS

485

Conclusions

RBS is a rapidly growing technique that has evolved from being used primarily in particle physics to being commonly applied and widely available. The size and cost of some RBS instruments are now equal to or less than that of other depth profiling techniques, such as SIMS, AES, and SNMS. RBS data analysis s o h a r e allows most data to be rapidly and accurately analyzed and permits the automated acquisition of and (in some cases) analysis of data. Currently RBS is used primarily in the analysis of semiconductors,superconductors,optical coatings, and other thin films. Some applications have been developed for polymers and ceramics, and hrther growth is expected in these areas due to the technique’s relatively lenient vacuum requirements and its insensitivity to charging problems for insulators. A few microbeam RBS systems are currently in service and the development of RBS imaging will certainly produce new applications for semiconductors and, possibly, even for biological samples, since the small size of cells that are typically analyzed has limited the use of RBS in the past. Related Articles in the Encydopedia

MEIS, ISS, PEE, EM, and NRA References 1

2

3

4

5

6

S. J. Valenty, J. J. Chera, G. A. Smith,W. Katz, R Argani, and H. Bakhru. J Polynter Sci. (Cbem.)22,3367,1984. S. M. Baumann, M. D. Strathman, and S. L. Suib. Analytical Cbem.60, 1046,1988. W. K. Chu, J. W. Mayer, and M. A. Nicolet. BackscatteringSpectrometry. Academic Press, New York, 1978. This is a frequently used handbook that provides a thorough discussion of the technique. J. R Bird and J. S. Williams. Ion Beamfir MaterialsAna&s. Academic Press,Australia, 1989. Chapter 3 provides an overview of RBS, while Chapter 6 reviews channeling techniques. This book also reviews N M , PME, SIMS, and other related ion-beam analyses. L. C. Feldman, J. W. Mayer, and S. T. Picraux. Materials Analysis by Ion Channeling.Academic Press,New York, 1982. This book provides an indepth study of the principles and use of ion channeling for analyzing materials. Channeling. (D. V. Morgan, ed.) John Wiley & Sons, London, 1973. Chapters 13-1 6 provide information regarding the use of channeling

486


Chapter 9

measurements in the analysis of materials. The remainder of the book is a study of the physics of channeling. 7

8

9

9.1

J. E Ziegler, J. I? Biersack, and U. Littmark. StoppingPowersandRangesin All Elements. Pergamon Press, New York, 1977, vol. 1-6. D. K. Sadana, M. Strathman, J. Washburn, and G. R Booker. App. Pbys. Lem. 37,234,1980. Ion Beam H a n d b o o ~ ~ r ~ a t e ~ a I A(J. n W. a ~Mayer s ~ . and E. Rimini, e&.) Academic Press,New York, 1977. This book provides useful tabular and graphic data for RBS,channeling, PKE, and NRA.

RBS

487

9.2

ERS Elastic Recoil Spectrometry J.E.E. BAGLIN

Contents

Introduction Principles of ERS Practical Considerations Conclusions

Introduction Overview

Elastic recoil spectrometry (ERS) is used for the specific detection ofhydrogen (‘H, 2H)in surface layers of thickness up to approximately 1 pm, and the determination of the concentration profile for each species as a function of depth below the sample’s surface.” When carefully used, the technique is nondestructive, absolute, fist, and independent of the host matrix and its chemical bonding structure. Although it requires an accelerator source ofMeV helium ions, the instrumentation is simple and the data interpretation is straightforward. The method may be contrasted to dynamic SIMS analysis, which, although capable of somewhat better depth resolution, is slower and matrix-dependent, and relies on ion milling (sputtering) for profiling. Nuclear Resonance Reaction Analysis (NRA)claims, in general, a better ability to identify trace (ppm) hydrogen levels, mainly because of enhanced (resonant) scattering cross sections. However, a depth profile determination by NRA is complex, requiring many sequential data runs, and it takes many times longer than EM. Quantitative NRA data reduction is 488

ION SCAlTERtNG TECHNIQUES

Chapter 9

Figure 1

The forward scattering concept of ERS.

hampered by the difficulties of determining and deconvoluting the nuclear resonance shapes. ERS may be regarded as an extension of Rutherford Backscattering Spectrometry It requires basically identical equipment, and it preserves many beneficial features of RBS: convenience, speed, precision, and simplicity. RBS is based on the simple point-charge scattering of ions (generally helium at 1-2 MeV) by the constituent atomic nuclei of the sample. The energy of ions scattered at a known angle is used to indicate both the mass of the scattering nucleus and the depth of penetration of the ion into the sample before the scattering collision occurred. T o optimize mass resolution and sensitivity, those ions which are scattered backwards (near 180') are frequently chosen for RBS spectrometry. This geometry does not work, however, when the projectile is heavier than the target nucleus. As illustrated in Figure 1, following the collision of a helium ion with a hydrogen (or deuterium) nucleus in the sample, both particles move in the forward direction, and it is therefore necessary to place detectors to receive forward scattered particles. Since most specimens will be too thick to allow either 4He or H ions to escape in transmission geometry, a glancing angle arrangement is chosen. In this situation, it is advantageous to select the recoiling hydrogen ions themselves for energy spectrometry, rather than the lower energy, less-penetrating scattered helium ions. By covering the detector with a stopping foil of appropriate thickness, it is possible to admit H ions for analysis but exclude all scattered He ions, including the prolific He ion flux contributed by Rutherford scattering from other, heavier constituents of the sample. The covered detector thus provides an energy spectrum of the forward-recoiling hydrogen ions. An important advantage of this technique is the uncomplicated relationship between this spectrum and the concentration-versus-depth profile for hydrogen in the sample. Derivation of the concentration profile is direct and unambiguous. This simplicity depends on the He-H scattering process being elastic (no residual excited states of scattered nuclei), and on the absence of nuclear reactions that might yield spurious detectable particles. The threshold energies for 9.2

ERS

489

such reactions are 6.7 MeV (for 4He + 2H +4He + n + ), and tens of MeV for 'H. Typically, ERS measurements are run with 1-2 MeV He ions, where such prob-

H

lems do not arise. Although most applications of ERS have used 4He as the projectile ion, the principle clearly can be extended to recoiling ions from heavier projectiles. The depth resolution may be significantly improved in this way, e.g., in polystyrene, the resolution found4 for ERS from 2.8-MeV He ions was 1000 compared with 300 obtained by using 20-MeV Si ions. Also, the scatteringcross section is larger for Si, leading to greater sensitivity. Severe radiation damage to samples can occur with heavy ions, however (hctors of >lo0worse than for He). In the interest of simplicity, this review will focus on the technique of hydrogen detection using helium ion beams.

a

Applications

ERS is an appropriate tool for a wide range of analytical applications. Some typical examples include the quantitation of hydrogen in glassy carbon films, the study of the dynamics of polymeric molecule^,^ studies of interface interactions between polymer films (using deuterium as a diffusion marker): analysis for hydrogen in natural geological specimens? a study of stress-induced redistribution of hydrogen in metal films? and a study of the effects of hydrogen content upon the optical, mechanical, and structural properties of plasma-deposited amorphous silicon and silicon nitride Further applicationswill also be found in the specific examples cited in this article.

Principles of ERS Apparatus

A typical experimental equipment layout for ERS is shown schematically in Figure 2. A beam of 4He ions is produced at an energy of 1-2 MeV, generally using an electrostatic accelerator and a beam transport system including a mass- and energy-resolving bending magnet and knife edge collimators to define the beam spot size and position. A typical beam current required is 5-50 nA, with a beam spot dimension of 1-2 111111. A clean vacuum of Ilo-' Torr is desirable for particle spectrometry, and specimens must be vacuum compatible. The specimen is tilted so that the incident beam makes an angle of approximately 15" with the plane of the surface, and a surface barrier detector is placed to receive particles scattered at a similar angle from the sample surface in the forward direction. The detector's aperture is set to limit the range of accepted scattering angles to f 1O or less. A smooth foil of aluminum or Mylar is placed in front of the detector to stop scattered He ions, yet to transmit scattered 'H (or 2H) ions into the detector after they incur a small, well-defined energy loss. It is important for this foil to be uniform and free of 490


Chapter 9

Cryopumped vacuum chamber

2 - 3 MeV

detector for RES /Absorber foil 10 micron mylar

Figure 2

LAperture

Typical experimental layout for ERS (schematic).

pinholes. A 10-pn Mylar foil will completely stop He ions of energy 5 2.3 MeV, while transmitting recoiling ‘H ions with an energy loss of 250 keV. An energy spread of I50 keV in the transmitted ions will be caused by ~traggling.~ A second surface barrier detector normally is used in a backscattering position. Spectra accumulated simultaneously with this detector will provide RBS information on the sample’s composition which must be combined with the ERS data for a complete sample analysis. Quantitative analysis requires a reliable measure of the incident ion fluence for each run. As in RBS, this may be obtained in various ways, either by direct collection of ion beam charge on the sample and in a surrounding Faraday enclosure (as illustrated in Figure 2), or by a well calibrated beam current sampling technique. In the former technique, complete collection of secondary electrons from the sample surface must be assured. A cooled sample stage mounted on a goniometer provides the means to adjust the sample’s orientation and, ideally, to raster the sample, in order to limit local ion beam heating. The sample stage may be cooled with liquid nitrogen to reduce the effects of beam-induced damage on the analysis, especially for polymers. Signals from the two surface barrier detectors are sorted by a pulse height analyzer, and the resulting spectra are stored for subsequent display and quantitative analysis. Typical run times of 15-20 minutes are required to obtain good statistics. It should be noted that this technique responds exclusively 9.2

ERS

49 1

/

a

-

C

-w

dq(h. t

Charnel width. AE

0

ca

l---

.!

0

E, (MeV)

Figure3

H ion energy, E,

(a) Scattering geometry for ERS; (b) ERS spectrum from 200-A partially deuterated polystyrene on Si, €, 3.0 MeV (adapted from ref. 101; and (c) schematic ERS depth profile sp-um.

to the presence of ‘H or 2H(or3H or 3He) nuclei in a sample, and does not suffer from any “background” from other sources. The sensitivity of the technique to trace amounts of H is therefore largely determined by patience and counting statistics. Energy Scale of ERS The energy scale of an elastic recoil spectrum provides information about the mass of the recoiling species, and about the depth within a sample at which the scattering took place. Figure 3a shows the ERS scattering geometry. To generalize the treatment, the fbllowingidentities are adopted: $ is the angle of hydrogen recoil with respect to the incident beam; M1and El are the mass and energy, respectively, of the incident ion; M2is the mass of the recoiling ion; E; is the energy of the recoiling particle at the point of the collision. 492

ION SCATTERING TECHNIQUES

Chapter 9

For the simple case of surface scattering (or scattering from a very thin layer), the ratio of 4 to El is given by E2--

4 Ml M2cos '$

El

(M1+M2I2

This ratio is known as the kinematic factor K'. The separation obtainable between 'H and 2H is evident (K'(lH) = 0.64 cos2$; K'(2H) = 0.88 cos2$), as seen in the energy spectrum of detected particles shown in Figure 3b, which was obtained from a 200-8, film of deuterated polystyrene on a silicon substrate." In practice, $ is generally set in the range of 10-30". When a stopper foil of thickness Gf(atom/ cm2) is used, the hydrogen energy observed at the detector is not actually &, but a lower value, Eobs,where EobS= E -6 S 2

ff

where Sfis the "stopping power" for hydrogen ions in the foil at the appropriate energy; i.e., the energy lost per unit thickness (atom/ cm2) of foil material. Obviously, it is best to choose the smallest 6f consistent with total stopping of unwanted 4He ions. In the more complex situation, a helium ion penetrates to a depth t (normal to the surfice) into the sample before encountering a hydrogen nucleus, where scattering occurs. The helium ion loses energy in ionizing collisions before the scattering event, and in turn the recoiling H must undergo a similar energy loss before escaping the sample's surface.If the energy-averaged stopping powers for 4He and recoiling H within the sample are Sl and &, respectively, then the depth tof the recoiling collision may be derived from the expression"

Generally S,

a 4

P

z

0

‘2 to

L 60

MASS (amu)

Figure 4

BO

IO

Mass scan acquired+om a zirconia crystal.

crystal (geological sample). It shows peaks for many elements and molecules, but provides no information concerningthe depth or lateral distribution of these impurities. Bulk Analysis Mode

Bulk analysis mode is typically used to obtain the lowest possible detection limits of one or several elements in a uniform sample. This mode of operation is similar to a depth profile with the sputtering rate set to the maximum. This causes the crater bottom to lose its flatness and allows impurities from the crater walls to be measured, thereby sacrificing depth resolution. Therefore, accurate measurement of impurities is obtained only when they are uniformly distributed in the sample. This method of measurement usually results in at least a factor-of-10 improvement in detection limits over the depth profiling mode. As an example, the detection limit of boron in silicon using the bulk analysis mode is 5 x 10’’ atoms/cm3, several orders of magnitude better than the boron background acquired using the depth profiling mode (6 x lo’* atoms/ cm3), as shown in Figure 3b.

540

MASS AND OPTICAL SPECTROSCOPIES

Chapter 10

Imaging Mode

SIMS imaging is performed using one of two methods. The first, called ion microscopy or stigmatic imaging, is only possible using specially constructed mass spectrometers capable of maintaining the x-y spatial relationships of the secondary ions. These mass spectrometers are typically specially configured double-focusing magnetic-sector spectrometers and are actually better termed secondary ion microscopes. The lateral resolution of microscope imaging is typically no better than 1 pm. The second method, scanning imaging, is performed by measuring the secondary ion intensiry as a function of the lateral position of a small spot scanning ion beam. The lateral resolution of this type of imaging is largely dependent on the diameter of the primary ion beam, which can be as small as 50 nm. Figures 5a and 5b are mass-resolved secondary ion images of gold (Au) and sulfur (S) in a cross-sectioned and polished pyrite (gold ore) sample acquired using the microscope imaging method. The gray level is proportional to the secondary ion intensity measured at each location, i.e., more gold or sulfur is found in darker locations. These images show that the gold, the geologist’s primary interest, is localized in the outer few Fm of the sulfur-containingpyrite grain. By acquiring mass-resolved images as a function of sputtering time, an imaging depth profile is obtained. This combined mode of operation provides simultaneous lateral and depth resolution to provide what is known as three-dimensional analysis. Sample Requirements

Most SIMS instruments are configured to handle samples less than 2.5 cm in diameter and 1 cm in thickness. The surface of the sample must be as smooth as possible because surface roughness causes a significant loss in depth resolution; cross sections and other cut samples must be well polished before analysis. In SIMS instruments capable of stigmatic imaging, the sample should be planar, because it effectively is part of the secondary ion optics. Nonplanar samples are better analyzed using a quadrupole SIMS instrument (discussed below) in which the sample shape does not affect the results as strongly. Samples composed of materials that are dielectric (nonconducting) must be analyzed using special conditions (see below). Quadrupole SIMS instruments are also less affected by sample charging and are often used to analyze dielectric samples. Artifacts

Although SIMS is one of the most powerll surface analysis techniques, its application is complicated by a variety of artifacts.

10.1

Dynamic SIMS

541

Figure 5

Mass-resolvedsecondary ion images of sulfur and gold in a pyrite ore sample. A comparison of the two images clearly shows the gold is found in the outer several pm of the sulfur-containingpyrite grain. These images were acquired using a magnetic-sectormass spectrometer in the microscope-imagingmode.

Mass Interferences

The most frequent artifacts arise from interferences in the mass spectrum, that is, ionized atomic clusters (molecular ions) or multiply charged ions whose nominal mass-to-charge ratio equals that of the elemental ions of interest. Such interferences can cause erroneous assignment of an element not present in the sample or simply can degrade the detection limit of the element of interest. Figure 6 is a mass spectrum obtained from high-purity silicon, using oxygen ion bombardment. In addition to the "Si+, 29Si+,and 30Si+isotope peaks, there exist numerous other peaks of atomic and molecular ions typically composed of primary ion species (oxygen), ions

542


Chapter 10

THE MASS SPECTRUM

o

to

20

u)

90

so

60

70

ao

90

io0

IW)

120 is0

HO I50

YaSr/Charg*

Figure 6

Typical secondary ion mass spectrum obtained from high-purity siiicon using an oxygen ion beam. Major ion peaks are identified in the spectrum.

of the major components of the sample (silicon), or atmospheric species (hydrogen, carbon, oxygen, nitrogen, etc.) remaining in the high-vacuum sample chamber. Many of these peaks are sufficiently intense to produce a measurable background, which may preclude determination of a specific element (impurity), wen in the PPm range. Once identified, voltage offsetand high mass resolution techniques may be used to reduce the detrimental effect of these interferingions. In the voltage o&et technique, the mass spectrometer is adjusted to accept only ions in a certain (usually higher) kinetic energy range. This technique is effective in discriminating against molecular ions because the energy distribution of atomic ions (typically the ions of interest) is broader than that of molecular ions at the same nominal mass. Figure 7 shows two SIMS depth profiles of the same silicon sample implanted with arsenic (75As).These depth profiles were obtained under normal conditions (0-V offset) and under voltage offset conditions (50-V ofiet). The improvement in the detection limit of arsenicwith the use of a 50-Vo k results from discriminationof the 29Si30Si'60 molecular ion dso at mass 75. High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of 30Siand 'H to form the molecular ion 3oSi1H-,severelydegrades the detection limit ofphosphorous r l P ) in a silicon sample. The exact mass of phosphorous (31P> is 31.9738 amu while the real masses of the interfering 30Si1Hand 29Si1Hz molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass 10.1

Dynamic SIMS

543

10'

-

lo5

"As+

0 YO115 OffS.1

10.

10

*

10

10'

100

Figure 7

Depth profile of an arsenic (75As1ion implantin silicon with and without use of vottage offset techniques. Voltage offset provides an enhanced detection limit for As in Si.

spectrum obtained from a phosphorus doped amorphous silicon thin film using high mass resolution techniques. The two mass interferences, 30Si1H- and 29Si1Hz-, are completely separated from the 31P peak. Quadrupole instruments are not usually capable of such high mass resolution. Primary Ion Beam Sputtering Equilibrium

As explained above, the mixing zone contains a mixture of atoms from the primary ion beam and the solid sample. In the case of oxygen or cesium ion bombardment, these primary species become part ofthe material in the mixing zone and can significantly alter the ion yields of elements in the sample. However, when sputtering is first started (at the beginning of a depth profile), the mixing zone contains very few atoms from the primary ion beam, causing ions ejected from the mixing zone to be less A c t e d by the enhancement process. Crater Bottom Roughening

In polycrystalline solids or samples consisting of various phases, each grain may sputter at a different rate producing extensive roughness in the bottom of the cra-

544


Chapter 10

P-clopad amorphous-Si film iHIW MASS RESOLUTION)

8

l

SlH

2a SiH 2

I

'

'

I

~

'

.

~

I

~

~

~

~

I

~

.

~

~

I

'

~

~

~

ter.3 The roughness of the crater bottom will result in a loss in depth resolution and cause the depth profile to appear smeared in depth. Surface Oxide

By enhancing the positive ion yield of most elements, the presence of an oxide on the suhce of a sample can cause the frrst several points of a SIMS depth profile to be misleadingly high. Exceptionally large secondary ion signals for most elements are observed while profiling through the surface oxide, even though their concentrations are not higher. In these cases, the first several data points are corrected or simply disregarded.

10.1

Dynamic SIMS

545

1

~

~

1

Sample Charging

The charge carried by positive primary ions can accumulate on the surface of a nonconducting sample, causing the primary ion beam to be defocused or to move away from the analytical area of interest, thus preventing continued analysis. In addition, the accumulated charge can change the energy of the ejected ions, thereby affecting their transmission and detection in the mass spectrometer.This effect, called sample charging is eliminated or reduced by flooding the sample su&e with a low energy electron beam, providing compensation for the build-up of positive charge. As a general rule, samples with resistivities above 1O8 ohm/ cm2 require the use of electron flooding. In highly insulating samples, the use of a negative primary ion beam may also alleviate this charging problem. Adsorption of Gaseous Species

During the sputtering process, residual atoms and molecules in the vacuum above the sample surface (typically containing hydrogen, carbon, nitrogen, and oxygen) are incorporated into the mixing zone by absorbing onto newly exposed and unsputtered reactive ions and molecules of the sample. The incorporated atmospheric species are eventually ejected from the mixing zone as elemental and molecular ions and detected as if they were originally present in the sample, complicating SIMS detection of these species and adding interfering molecular ions to the secondary ion mass spectrum. As an example, a mass interference between 31P, and 30Si’H and 2’SiH2, all having mass 31,can be caused by hydrogen from the atmosphere in the sample chamber. The detrimental effects of these atmospheric species can be reduced by improving the vacuum in the sample chamber, but no matter how good the vacuum is, some adsorption will occur. Impurity Mobility-lon Beam-Induced Diffusion

Another difficulty is ion beam-induced diffusion of extremely mobile ions, such as lithium and sodium, in dielectric thin-film samples. This efict is normally observed when depth profding a dielectric thin film on a conducting substrate with a positive primary ion beam. Dihsion occurs because the primary ion beam deposits a charge on the sample surface, creating a large electric field across the thin film, thereby driving the mobile ions away from the surface, to the intedace between the thin film and substrate. In bulk insulators, this problem may be less severe because the electric field gradient is smaller. Nonetheless, the acquired depth profde no longer reflects the original composition of the sample. This effect is reduced or eliminated by flooding the sample surkce with a low energy beam of electrons during sputtering. The current of electrons striking the sample surface must be carefully balanced aginst the build-up of charge due to the primary ion beam. Otherwise, distortion of the depth profile will still occur.As a general rule, quadrupole mass spectrometers have much less difficulty with impurity mobiliry artifacts

546


Chapter 10

than do magnetic sector spectrometers, and they are almost always used in these applications. Quantification

Ion yields of different elements vary by several orders of magnitude and depend sensitively on the type of primary beam and sample. Accurate quantification requires comparison to standards or reference materials of similar or identical major element composition that must be measured using the same analytical conditions, especially using the same type of primary ion beam. For example, an aluminum sample with a known content of copper is not a good standard to use for quantification of copper in stainless steel. Similarly, a standard analyzed using a cesium primary ion beam must not be used as a standard for quantification of an unknown sample analyzed using an oxygen ion beam. In some cases,semiempirical ion yield systematics are successhlly used to quantify certain analyses; this method of quantification is accurate only to within an order of magnitude. Ion implantation is often used to produce reliable standards for quantification of SIMS ana lyse^.^ Ion implantation allows the introduction of a known amount of an element into a solid sample. A sample with a major component composition similar to that of the unknown sample may be implanted to produce an accurate standard. The accuracy of quantification using this implantation method can be as good as 52%. Instrumentation

SIMS instruments are generally distinguished by their primary ion beams, and the kinds of spectrometers they use to measure the secondary ions. Several types of primary ion beams-typically, oxygen, cesium, argon, or a liquid metal like galliumare used in SIMS analyses, depending on the application. Nearly any SIMS instrument can be configured with one or more of these ion-beam types. The majority of SIMS mass spectrometers fall into three basic categories: double-focusing electrostatic or magnetic sector, quadrupole, and time-of-flight. Time-of-flight analyzers are primarily used for surfice and organic analyses (especially for high molecular weight species) and are mentioned in the article on static SIMS. A double-focusing, electrostatic or magnetic-sector mass spectrometer achieves mass separation using an electrostatic analyzer and magnet. Secondary ions of different mass are physically separated in the magnetic field, with light elements making a tight arc through the magnet and heavy elements making a broad arc. Ions of different charge-to-mass ratios are measured by changing the strength of the magnetic field in the magnet to align the ions of interest with a stationary detector. Magnetic-sector systems provide excellent detection limits because of their high transmission efficiency, and are capable of high mass resolution. Some of these 10.1

Dynamic SIMS

547

spectrometers are capable of stigmatic imaging (also called ion microscopy) which is used to acquire mass-resolved ion images with a resolution as good as 1 p. In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. The choice of mass spectrometer for a particular analysis depends on the nature of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. Conclusions

SIMS is one of the most powerful surface and microanalytical techniques for materials characterization. It is primarily used in the analysis of semiconductors,as well as for metallurgical, and geological materials. The advent of a growing number of standards for SIMS has greatly enhanced the quantitative accuracy and reliability of the technique in these areas. Future development is expected in the area of small spot analysis, implementation of post-sputtering ionization to SIMS (see the articles on SALI and SNMS), and newer areas of application, such as ceramics, polymers, and biological and pharmaceutical materials. Related Articles in the Encyclopedia

Static SIMS, SALI, SNMS, and Surface Roughness References 1

2 3

4

G. Slodzian. Su$Sci. 48, 161, 1975. C. A. Andersen. Int. J,Mms. Spect. Io Phs. 3,413, 1970. E. Zinner, S. Dnst, J. Chaumont, and J. C. Dran. Proceedinp of the Ninth Lunar and Planeta y Sciences Conference. 1978, p. 1667. I? Williams. IEEE Trans.NucI. Sci. 26, 1809, 1979.

548


Chapter 10

Static SlMS

10.2

Static Secondary Ion Mass Spectrometry BILL KATZ

Contents

Introduction Basic Principles Sample Preparation Instrumentation Qualitative Analysis QuantitativeAnalysis Chemical Mapping Conclusions

Introduction

With today’s technology, the definition of the surface as it effects a material’s performance in many cases means the outer one or two monolayers. It is the specific chemistry of these immediate surfice molecules that determines many of the chemical and physical properties. Therefore, it is important to have availablea tool that is able to characterize the chemistry of these layers. One such method that has met with considerable success is Static Secondary Ion Mass Spectrometry (SIMS). Static SIMS entails the bombardment of a sample surface with an energetic beam of particles, resulting in the emission of surface atoms and clusters. These ejected species subsequently become either positively or negatively charged and.are referred to as secondary ions. The secondary ions are the actual analytical signal in SIMS. A mass spectrometer is used to separate the secondary ions with respect to their charge-to-mass ratios. The atomic ions give an elemental identification (see 10.2

Static SlMS

549

the article on dynamic SIMS), whereas the clusters can provide information on the chemical groups. The mass spectrum of the clusters obtained represents a fingerprint of the compounds analyzed. The data show the various chemical functional groups as they fragment during analysis. The data may be acquired over relatively small areas (pm or less) for a localized analysis or larger areas (mm) for a macrocharacterization. Further, by monitoring a particular charge-to-mass ratio (i.e., a particular chemical group), one can obtain chemical maps depicting the lateral distribution of a specific fragment or compound.

Basic Principles

The analytical signal (he secondary ions) is generated by the interaction of an energetic particle with a sample surface. This interaction can be divided into two regimes by the total flux of primary particles used. In what is known as Dynamic SIMS, the incident flux of primary ions is large enough (above .512 atoms/cm2) that, statistically, a given area has a high probability of being repetitively bombarded, causing a crater to be formed in the sample. This mode of SIMS provides an in-depth analysis of any element (including H) and its isotopes with excellent detection limits (ppm to ppb atomic). The disadvantage is the large primary fluxes tend to alter or totally obliterate the chemistry, limiting dynamic SIMS to elemental analysis. When the total flux is kept below 512 atoms/cm2, the probability of a primary ion striking a previously analyzed area is low, thus leaving the surface chemistry intact. This is the mode of operation used for Static SIMS. With respect to the primary beam, the total incident flux is the critical parameter for maintaining the chemical integrity of the material. Typically, static SIMS is performed using an inert gas beam operated at kinetic energies between 1 and 10 keV. The beam can be composed either of positively charged ions (AI-+, Xe+)or neutral particles (Ar,Xe) depending upon the type of ion source used. In the SIMS process the energy of the incoming primary particles is dissipated by collisional cascades within the sample as the primary penetrates well into the rnaterial. The energy transferred to the sample is sufficient to cause atoms and molecules at the surfice (within 1-3 atomic layers) to be ejected. A small fraction of these carry positive or negative charge, and are detected. In static SIMS, since one is primarily after chemical information, the identification of the cluster molecular ions is of particular interest as it allows one to identiG the chemical compounds present at the surface. Unfortunately, in many-atom compounds severe fragmentation of the dusters takes place during the ejection and ionization process, with the parent ions rarely being observed. In most cases, therefore, identification of the parent species must rely on deductions made from the fragmentation patterns observed. 550


Chapter 10

Sample Preparation

A major advantage of static SIMS over many other analytical methods is that usually no sample preparation is required. A solid sample is loaded directly into the instrument with the condition that it be compatible with an ultrahigh vacuum (10-9-1 0-l' torr) environment. Other than this, the only constraint is one of sample size, which naturally varies from system to system. Most SIMS instruments can handle samples up to 1-2 inches in diameter. In specialized cases, a treatment known as cationization sometimes is tried to improve the amount of molecular (chemical) information made available. If Ag or Na are deliberately introduced into the sample, they will often combine with the molecular species present to create Ag+ or Na' molecular ions. These ions are more stabie to fragmentation than the bare molecular ions, and can therefore be observed more easily in the mass spectrum. The identification of parent ion peaks in this manner aids in detailed chemical identification. Static SIMS is also capable of analyzing liquids and fine particles or powders. A liquid is often prepared by putting down an extremely thin layer on a flat substrate, such as a silicon wafer. Particles are easily prepared by pressing them onto doublesided tape. No further sample preparation, such as gold- or carbon-coating, is required. Because of the extreme surface sensitivity of static SIMS, care should always be exercised not to handle the samples. Clean tools and gloves should be used always to avoid the possibility of contaminating the surfice. While it is possible to remove surface contamination with solvents like hexane, it is always desirable not to clean the surface to be analyzed. Instrumentation

All commercially available SIMS systems have in common some type of computer automation, an ion source, a high-vacuum environment, and some type of mass spectrometer. While the specifics may vary from system to system, the basic requirements are the same. The hardware feature that tends to distinguish the various systems is the type of mass spectrometer used. These fall into three basic categories: 1 Quadrupole spectrometers. These are the least expensive mass spectrometers, and the easiest to operate. By applying AC and DC potentials to a set of four rods, ions are separated by mass as they pass through the quadrupole. The voltages can be changed quickly, allowing relatively rapid scanning of the mass range, which is usually limited to around 1000 m u . Because quadrupoles cannot effectively separate ions having a wide energy spectrum, an electrostaticfilter is used between the sample and the quadrupole. Perhaps the major drawback to 10.2

Static SIMS

551

quadrupoles is that they have only moderate mass resolution and cannot separate peaks of the same nominal mass. Magnetic-sector spectrometers.These spectrometersuse an electrostaticanalyzer for energy filtering and a magnetic sector for mass separation. They are capable of achievinghigh mass resolution, with typical mass ranges of 250 mu. Time-of-flight spectrometers. Eme-ofjZight (TOF) analyzers are capable of both high mass resolution and extended mass range. Their design requires that the ion beam be pulsed (1-10 ns) to ensure high mass resolution. After the ion beam strikes the sample surface, the extracted secondary ions travel through a drift tube to a detector having a large area. Mass separation is accomplished by noting that ions having different masses take diffefent times to reach the detector. For example, lighter secondaryions take less time to traverse the drifc tube and reach the detector hter. Using this type of mass analyzer enables the entire mass spectrum to be collected in a few ps. Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fiirly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surfice analytical method, such as electron spectroscopy for chemical analysis or scanningAuger spectroscopy. Contrasted with the quadrupole filter, both magnetic-sector and TOF analyzers provide the advantage of high mass resolution. This enables the separation of fragments having similar masses into distinct peaks. These instruments are useful when dealing with analytical situations requiring more exact mass determination. If there is a requirement to measure to high mass (thousands of amu), such as may be the case when studying polymers, TOF analyzers become the only choice. Qualitative Analysis

One of the most common modes of characterization involves the determination of a material’s surface chemistry. This is accomplished via interpretation of the fragmentation pattern in the static SIMS mass spectrum. This “fingerprint” yields a great deal of information about a sample’s outer chemical nature, including the relative degree of unsaturation, the presence or absence of aromatic groups, and branching. In addition to the chemical information, the mass spectrum also provides data about any surface impurities or contaminants. Figure 1 shows a positive static SIMS spectrum (obtained using a quadrupole) for polyethylene over the mass range e200 m u . The data are plotted as secondary ion intensity on a linear y-axis as a function of their charge-to-mass ratios (amu). This spectrum can be compared to a similar analysis from polystyrene seen in Figure 2. One can note easily the differences in fragmentationpatterns between the 552


Chapter 10

10

0 8 7

6 5 4

3 2 1

0 0 Figure 1

MASS/CHARGE, m/e Positive mass spectrum from polyethylene, 0-200 amu.

two materials. Polystyrene is seen to have distinct fragment peaks characteristic of the aromatic nature of the compound. With the exception of the large peak at mass 91, most of the interesting information is found above 100 m u . Fragment peaks between 100 and 200 amu can be assigned to structures corresponding to rearrangements of the polystyrene backbone with one or more phenyl groups attached. This is seen to be dramatically different fiom the polyethylene spectrum, which is typical for a saturated short-chain hydrocarbon, with no significant peaks above 100 amu. This is a typical example of the current use of SIMS.

.-

an-

MASWCHARGE, m/e Figure 2

10.2

200

Positive mass spectrum from polystyrene, 0-200 amu.

Static SIMS

553

Polyphenylene rulfide (+)

+a

200

MASS/CHARGE, d e Figure 3

Positive mass spectrumfrom polyphenylene sulfiie, W O O amu.

As mentioned earlier, the secondary ions ejected from a&s can be positively or negatively charged. Analytically, this is quite useful, as certain species are more readily ionized in one mode than another. The positive and negative mass spectra from polyphenylene sulfide shown in Figures 3 and 4, respectively, illustrate this point. The positive mass spectrum looks quite similar to that of polyethylene (Figure l),with no indication of either the phenyl ring or the sulfur atom found in the polyphenylene sulfide. In marked contrast, the negative spectrum in Figure 4 clearly shows the presence of the phenyl group with the sulfur atom attached. This demonstrates how both positive and negative spectrometry may be needed to M y characterize a structure.

.p - s -

Polyphenylene sumde (-)

+@a

.9

+n

s

$

4

$

3 2 1

a

Figure 4 554

0

MASWCHARGE, m/e Negative mass spectrum from polyphenylene sulfide, W50 amu.


Chapter 10

2600 u)

-+

c

2

0

1300 r)

LL

0

c 10

0

20

ELLIPS. THICKNESS 8, Figure 5

Plot of positive CF, secondary ion intensity versus ellipsometric thickness

from a set of perfluoropolyether standards.

Quantitative Analysis

As with any analytical method, the ability to extract semiquantitativeor quantitative information is the ultimate challenge. Generally, static SIMS is not used in this mode, but one applicationwhere static SIMS has been used successfilly to provide quantitative data is in the accurate determination of the coverage of fluropolymer lubricants.’ These compounds provide the lubrication for Winchester-type hard disks and are directly related to ultimate performance. If the lubricant is either too thick or too thin, catastrophic head crashes can occur. Initially, a set of lubricant film standards of various thicknesses were prepared and their thickness measured by Fourier transform infrared spectroscopyand ellipsometry. Once good correlation between these two techniques was achieved, the same samples were analyzed using static SIMS. The CF, peak in the positive SIMS spectrum, which is characteristic of the fluoropolymer lubricant, was measured and its intensity plotted versus the thickness determined by ellipsometry. These results are presented in Figure 5, and show excellent correlation. It must be understood that since static SIMS analyzes the outer few monolayers, this measurement actually follows the coverage of the lubricant film. As more lubricant is put down, the coverage across the sample becomes more unifbrm, giving a higher secondary ion yield for the CF, fragment. Because of the excellent correlation between SIMS and other methods, one can conclude that all these techniques are actually measuring the effect of increasinglubricant coverage. However, static SIMS has been found to be more accurate for film thicknesses below 10 A, owing to its extreme surface sensitivity. In addition, one also obtains an analysis of any contamination from the complete SIMS spectrum. 10.2

Static SIMS

555

Another exampleof static SIMS used in a more quantitative role is in the analysis of extruded polymer blends. The morphology of blended polymers processed by extrusion or molding can be affected by the melt temperature, and pressure, etc. The surfice morphology can have an effect on the properties of the molded polymer. Adhesion, mechanical properties, and physical appearance are just a few properties affected by processing conditions. In a molded polymer blend, the surface morphology results from variations in composition between the surface and the bulk. Static SIMS was used to semiquantitatively provide information on the surfice chemistry on a polycarbonate (PC)/ polybutylene terephthalate (PBT) blend.3 Samples of pure PC, pure PBT, and PC/PBT blends of known composition were prepared and analyzed using static SIMS. Fragment peaks characteristic of the PC and PBT materials were identified. By measuring the SIMS intensities of these characteristic peaks from the PC/PBT blends, a typical working curve between secondary ion intensity and polymer blend composition was determined. A static SIMS analysis of the extruded surface of a blended polymer was performed. The peak intensities could then be compared with the known samples in the working curve to provide information about the relative amounts of PC and PBT on the actual surface. Chemical Mapping

In addition to data obtained using the spectral mode of analysis, it is often important to know the location of a particular chemical group or compound on the sample surface. Such information is achieved by static SIMS chemical mapping-a procedure in which a specific chemical functionality on the material is imaged, providing information as to its lateral distribution on the surface. The use of chemical mapping is demonstrated in the following example involving the delamination of a silicone primer and polytetrafluoroethylene (PTFE) material. The positive mass spectrum acquired from the delaminated interface contains peaks known to be uniquely characteristic of PTFE (CF3 at mass 69) and the silicone primer (Si(CH3)3at mass 73). Figures 6 and 7 are secondary ion images of the CF3 and (Si(CH& fragments taken from a 1-mm2 area of the delaminated interface. These maps dearly indicate that the PTFE and the silicone primer exist in well-defined and complementaryareas. Conclusions

Static SIMS has been demonstrated to be a valuable tool in the chemical characterization of surfaces. It is unique in its ability to provide chemical information with high surface sensitivity. The technique is capable of providing mass spectral data (both positive and negative spectrometry), as well as chemical mapping, thereby giving a complete microchemical analysis. The type of information provided by 556


Chapter 10

Figure 6

Positive ion image of CF, taken from a 1-mm2area.

Figure 7

Positive ion image of Si(CH,), taken from a 1-mm2 area.

static SIMS has been used to solve problems in a wide range of applications, including impurity analysis, the comparison of surface and bulk compositions, failure analysis, and the determination of adhesion or delamination mechanism^.*-'^ With the increasing availability of TOF instruments, the field will see more applications involving the analysis of higher molecular weight fragments. This, coupled with the higher mass resolving power of TOF systems, will open up research in such fields as biomedical and pharmaceutical applications, in addition to all areas in high-technology materials where the identification of contaminants 10.2

Static SIMS

557

of high amu in trace amounts at surfaces is important. Residues from previous processing steps are prime examples, both in semiconductorsand other thin-film technologies. Related Articles in the E myclopedia

Dynamic SIMS and SAL1 References

E A. White and G. M. Wood. Mass Spectromeq. John Wiley and Sons, New York, 1986. 2 J. G. Newman and K. V. Viswanathan. /. Vu.Sci. Tech A8 (3),2388, 1990. 3 R. S. Michael, W. Katz, J. Newman, and J. Moulder. Proceedings of the seventh International SIMS Conference. 1989,p. 773. 4 W. Katz and J. G. Newman. MRS Bulktin. 12 (G), 40,1987.Reviews the fundamentals of SIMS. 5 D. Briggs. Polymer. 25, 1379,1984.Review of static SIMS analysis. 6 A. Brown and J. C. Vickerman. Surf IntPrfaceAnal.6,1,1984.Describes interpretation of fragmentation patterns in static SIMS. 7 W.J. van Ooij and R H. G. Brinkhuis. Surf IntefaceAnal. 11,430, 1988.Discusses fingerprint patterns characteristic of the molecular repeat unit of a polymer. a R. S. Michael and W. J. van Ooij. Proc. ACS Diu. Polymer Mater. Sci. Eng. 59,734,1988.Static SIMS analysis of plasma treated polymer surfaces. s D. Briggs. Ox.Mass Spectrom. 22,91,1987.Static SIMS analysis of copolymers. IO M. J. Hearn, B. D. Ratner, andD. Brigp. Macromol. 21,2950,1988.Use of peak intensities in static SIMS for quantification. i

558


Chapter 10

SAL1

10.3

Surface Analysis by Laser Ionization S U S A N G . MACKAY A N D C H R I S T O P H E R H . BECKER

Contents

..

Introduction Basic Principles Common Modes of Analysis and Examples Instrumentation Conclusions

Introduction

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ionize rhe major neutral components ejected from surfices under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-InitiatedResonance Ionization Spectroscopy (SINS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy( X P S ) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. The advantages of SALI are seen most dearly when analyzingtrace (ppm to ppb) amounts of material on surfaces or ar interfaces. Typically, SALI analyzes the same samples as SIMS, with the added advantage of providing easily quantifiable data. 10.3

SAL1

559

Technique

SIMS

SAL1

Analysis modes

AES

XPS

RBS

Surface,

Surface,

depthprofiling, imaging

depthprofiling, imaging

Surface, depthprofiling, imaging

Surface, depthprofiling, imaging

Surface, depthprofiling

Common detection limit

ppm-ppb

ppm-ppb

0.05% at.

0.1% at.

2% at. -10 ppm

Probing depth

2-5

Ultimate spatial resolution

60 nm

GO nm

10 pm

100A

1 mm

Quantitative?

Yes

Semi,with

Yes

Yes

Yes

A

2-5

A

2-30

A

2-30

A

30-100 A

rigorous

standards Table 1

Comparison of SAL1to other surface spectroscopic techniques.

Whereas SIMS provides highly matrix-dependent data, SALI can resolve problems associated with SIMS ion-yield transients. SALI has been applied to two basic groups of samples: inorganic and organic solid materials. For inorganic analysis or elemental analysis (e.g., semiconductors and metal alloys), ionization by absorption of more than one photon (multiphoton ionization) is generally used as the postionization source, while for organic analysis (e.g., polymers and biomaterials), a less destructive single-photon ionization probe is employed. In order to provide lateral and depth information, SALI can be operated in both mapping and depth profiling modes. SALI compares favorablywith other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantage is the combination of analytical versatility, ease of quantification, and sensitivity. Table l compares the analytical characteristics of SALI to four major surface spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint infbrmation via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. X P S and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative; however, SALI molecular information can potentially be quantified directly without correlation with another surface spectroscopic technique. A E S and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield little structural infbrmation. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 560


Chapter 10

SALI, X P S , and AES have a nearly uniform sensitivity for all detectable elements with respect to the chemical composition of the sample matrix. SIMS and RBS, however, have sensitivities that vary greatly due to matrix effects. For RBS the matrix dependence is well understood, with the best sensitivities (10 ppm) found for heavy elements in light matrices while the worst sensitivities (3% at. to undetectable) are seen for light elements in heavy matrices. SIMS sensitivity varies for elements depending on both the chemical composition of the sample and the composition of the primary ion beam. Common primary ion beams are fbr SIMS 0 2 + and Cs+each of which enhances secondary ion yields. @n, an advantage of SALI is its relative insensitivity to the effects of changing matrix composition. Two drawbacks of SALI are that its maximum sensitivity is usually less than optimum case SIMS and, like all sputtering techniques, it is destructive. A somewhat related technique is that of laser ionization mass spectrometry (LIMS), also known as LIMA and LAMMA, where a single pulsed laser beam ablates material and simultaneously causes some ionization, analogous to samples beyond the outer surfice and therefbre is more of a bulk analysis technique; it also has severe quantifiaction problems, often even more extreme than for SIMS. Basic Principles

Figure 1 is a schematic of the SALI process. An energetic probe beam (ions, electrons, or light) is used to desorb material from the sample’s surface. The neutral component is then intersected and ionized by a high-power, focused laser beam. The laser beam is passed parallel and close to the surface so that it intersects a large solid angle of the sputtered neutral species, thus improving the sensitivity. The positive ions created by the laser light (photoions) are then extracted by a high-voltage field (>3 kv) and pass into the mass analyzer. A time-of-flight tube is used to mass analyze the photoions created by each pulse of the laser. Lighter ions have a shorter flight time than heavier ions and therefore arrive at the ion detector (channel plate assembly) sooner. A typical SALI analysis looks at a mass range per laser pulse of approximately m/ z = 12-300 amu (mass spectra fingerprint region) with a totd acquisition time of 5 seconds (1000 pulses with a 200-Hz laser). Analysis of Neutrals

Previous studies of the interaction of energetic particles with surfaces have made it clear that under nearly all conditions the majority of atoms or molecules removed from a surface are neutral, rather than charged. This means that the charged component can have large relative fluctuations (orders of magnitude) depending on the local chemical matrix. Calibration with standards for surfaces is difficult and for interfaces is nearly impossible. Therefore, for quantification ease, the majority neutral component of the departing flux must be sampled, and this requires some type of ionization above the sample, often referred to as post-ionization. SALI uses effi-

10.3

SAL1

561

l i m e - of - Right O F ) Mass Analyser

Figure 1

Schematic representation of the fundamental SAU process.

cient, nonresonant (not tuned to a specific energy level), and therefore nonselective photoionization by pulsed untuned laser radiation to accomplish this ionization and thus make this detection scheme a reality. The mass spectrometer, not the laser, performs the chemical differentiation. The commercial availability of intense laser radiation with high brightness makes this technique viable. Photoionization

Two forms of nonselective photoionization have been used for SALI, one primarily for elemental analysis and the other primarily for molecular analysis. For elemental analysis, a powerful pulsed laser delivering focused power densities greater than 10" W/cm2 is used for multiphoton ionization (MPI). This typically ionizes a l l the species within the laser focus volume without the need for wavelength tuning and regardless of chemical type. This nonresonant photoionization yields the desired uniformity of detection probability essential for quantification. For molecular analysis, a soft (;.e., nonfragmenting) photoionization is needed so that the mass peaks in the mass spectrum correspond to larger chemical units. This is supplied by vacuum ultraviolet light with wavelengths in the range 115-120 nm (1011 ev). Photoionization of this type is achieved by single-photon ionization (SPI). 562


Chapter 10

Relative photoionization cross sections for molecules do not vary greatly between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionization methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles; this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. Time-of-flightMass Spectrometry (TOFMS)

The advantage in using pulsed lasers is that they provide an excellent time marker for TOFMS. With TOFMS, a high mass resolution of several thousand can be achieved by energy focusing using a simple reflecting device, the instrument transmission is exceptional; and there is a multiplex advantage in mass. With the multiplex advantage, all masses are detected (paralleldetection) within an extremely high mass range (up to 10,000 atomic mass units or more). The mass multiplex advantage has a dramatic impact on the instrument's sensitivitywhen numerous elemental or molecular species are present-a very common occurrence. Surface Removal for Sampling

Surface removal for sampling involves removing atoms and molecules from the top surface layer into the vapor phase. The fact that the ionization step is decoupled from the surface removal step implies a great deal of flexibility and control in the types and conditions of the energetic beam of particles chosen to stimulate desorption. For elemental analysis of inorganic materials, typically a 50-pm Ar', or subpm diameter Ga' beam at several keV is used. Argon is used as an intense, high fluence ion beam that provides minimal chemical modification to the sample. Gallium is used as a liquid metal ion source that provides a highly focused, bright source for small area analysis (60-200 nm). Submonolayer or static analysis can be obtained by pulsing the beam and keeping the total dose extremely low (e l O I 3 ions/cm2). Depth profiling is accomplished by dc ion-beam milling and gating the pulsed photoionization to sample from the center of the sputter crater, which maintains state-of-the-art depth resolution. Ion-beam erosion is used to reveal buried interfaces during depth profiling, achieving a depth resolution often on the order of 20 W after sputtering 1 l m in depth. T h e small-spot Ga' beam is well suited for quatitative chemical mapping with sub-pm spatial resolution. For other material types, such as bulk polymers, using energetic electrons, or another laser beam sometimes results in superior mass spectra; these sources ofcen can remove dusters with less fragmentation, than pulsed ion beam sputtering and thus yield more characteristic mass peaks. For thermally sensitive samples, even thermal desorption can be used to investigate their temperature dependence. 10.3

SAL1

563

Common Modes of Analysis and Examples

SALI applies two methods of post-ionization, MPI and SPI, each of which can be used in one of the three modes of analysis: survey analysis, depth profiling, and mapping: 1 Survey spectra using the MPI method are used primarily for quantification of surface components in inorganic materials, with a detection limit ofppm to ppb. The same mode coupled with SPI can be used for molecular characterization of polymer films. 2

Depth profiling by SALI provides quantitative information through interfaces and for extremely thin films, in the form of reliable chemical concentrations.

3

SALI mapping is a sensitive and quantitative method to characterize the spatial distribution of elements in both insulating and conductive materials. Survey Mode

Surveys using MPI reveal the elemental composition of solid materials. Therefore this mode is employed most often in the analysis of inorganic materials like semiconductor devices and catalysts. Quantification can be achieved by using loosely matched standards and is accurate to within 10-20%. SPI has two advantages over MPI for the analysis of organic materials. First, it is a soft ionization method, so there is less fragmentation in addition to that of the primary beam, and second, the photoionization cross sections are nearly identical for molecules of similar size but different chemical type. This second characteristic enables SPI to provide semiquantitative raw data for all classes of organic materials without rigorous standards. Figure 2 is an example of a SALI mass spectrum of polyethylene glycol using SPI. The dominance of the monomer peak is an example of the simple molecular identification using this technique. Depth Profiling Mode

As stated above, SALI depth profiling is performed by gating the post-ionization beam by firing the laser only when the center of the crater is being sampled. This minimizes the contribution from the crater edge to the total signal at a specific depth, which increases the achievable depth resolution. Therefore, the depth resolution achieved by SALI easily equals that of SIMS which also employs gating. The major difference between these two depth profile techniques is that for SALI the sensitivity is nearly uniform for all elements, while for SIMS the sensitivity varies greatly. In selected cases this is an advantage for SIMS because the secondary ion yield for certain elements can be chemically enhanced, for example, by using a primary ion-beam composed of 0 2 + or Cs+. However, it also severely limits the ability to quantify SIMS data because secondary ion yields can vary by orders of magnitude depending on the chemical composition of the matrix or probe beam. This is 564


Chapter 10

10

3 8

7

6

5 4

3 2 1 0

20

40

60

00

100

120

140

SPI-SAU mass spectrum of a thin film of polyethyleneglycol. The major peaks are identified on the spectrum. Analytical conditions: 7-keV AP, 5 - p pulse length; 118-nm radiation.

Figure 2

a problem when analyzing thin films and elemental distributions across chemically dissimilar interfaces because the changing ion yield causes changes in the ion signal intensity. In these cases SIMS ion yield transients can severely distort a depth profile and can be resolved only by using rigorous standards. An example' is a depth profile of a F implant (1015F atoms/cm2 at 93 kev) in a 2000 A-thick polycrystalline Si sample on a thin Si02 layer on crystalline Si. Figure 3 is of an unannealed sample, where a smooth F distribution is expected. The SALI depth profile in Figure 3a shows the expected smooth distribution of the F implant. The SIMS data shown in Figure 3b, however, shows the common influence of matrix effects at an interface where the F positive ion yield is enhanced by the oxygen in the Si02 layer. The relative insensitivity of SALI to matrix effects is a tremendous advantage over SIMS in terms of quantitative depth profiling. Also, the usell yield (a measure of sensitivity) for the majority of elements falls into the range when using SALI range when using SIMS. Usell yield is defined as compared to the to e the number of ions detected versus the total material removed during analysis, and the efficiency of SALI can be equal to SIMS and orders of magnitude better than other nonselective post-ionization techniques (electron impact and radiofrequency low-pressure plasma). Mapping Mode

The determination of the lateral distributions of chemical species on surfaces is of constantly increasing technological importance in many applications, such as integrated circuit manufacturing. The two major tools that have been available are 10.3

SAL1

565

0

io00

2000 DEPTH (A)

Figure 3

Depth profiles of F implanted into 2000 A Si on SiOz: (a) SAL1 profile with Ar+ sputteringand 24&nm photoionization; and (b) positiveSIMS profile with Oz+ sputtering. Analytical conditions: (SALI, SiF profile) 7-keV AP, 248 nm; (SIMS, F profile) 7-keV 02+.

ScanningAuger Electron Spectroscopy (SAM) and SIMS (in microprobe or microscope modes). SAM is the most widespread technique, but generally is considered to be of lesser sensitivity than SIMS, at least for spatial resolutions (defined by primary beam diameter d) of approximately 2 0.1 pm. However, with a field emission electron source, S A M can achieve sensitivities ranging from 0.3% at. to 3% at. for dranging from 1000 to 300 respectively, which is competitive with the best ion microprobes. Even with competitive sensitivity, though, S A M can be very problematic for insulators and electron-sensitivematerials. The sensitivitiesfor SIMS are extremely variable, depending both on the species of interest and the local chemical matrix (so-called matrix effects). Quantification is very problematic for SIMS imaging because of matrix effects; on the very small scale associated with chemical imaging (sub-pn), it is not possible to generate closely matched reference materials because compositions change quickly and in an uncontrollableway. In the microscope mode, SIMS spatial resolutions are generally limited to about 1 pm. In the scanning mode, liquid-metal ion guns (notably Ga') have been used with better spatial resolution (sub-pm) but are somewhat unsatisfactory because Ga' is not effective for increasing secondary ion yields, unlike 02'

a

566

a,


Chapter 10

Figure 4

Chemical images of a nickel TEM grid. Field of view is approximately 25 x 15 pm, 50 x 50 pixels. Analytical conditions: Ga* sputtering, spot size about 0.2 pm, 248-nm radiation, acquisition time 33 minutes.

or Cs'. The sensitivity for scanning SIMS can range, for Lxample, from 0.01-10% at d = 1 pm (using 0 2 ' or Cs' for ionization enhancement), to 1% to undetectable at d = 0.1 pm (using Ga'). By examining the sputtered neutral particles (the majority channel) using nonselective photoionization and TOFMS, SALI generates a relatively uniform sensitivity with semiquantitative raw data and overcomes many of the problems associated with SIMS. Estimates for sensitivities vary depending on the lateral spatial resolution for a commercial liquid-metal (Ga') ion gun. Calculated values2 for SALI 10.3

SAL1

567

mapping show the sensitivity ranging from 0.2% to 3% at d = 1 to 0.1 p.These sensitivitiesrange as shown in Figure 4, which is a SALI image of a nickel TEM grid using Ga' sputtering and photoionization of the emitted neutrals at 248 nm (MPI, using KrF radiation). The pixel resolution achieved is < 0.5 w, while the spot size d of the Ga' beam was 0.2 pm. As work in this area progresses and state-of-the-art liquid metal ion guns are used, the lateral resolutions achieved should approach the expected values. While the acquisition time for the sample image was somewhat long (33 minutes) this represents initial work. The acquisition time can be decreased readily by a fictor of 10 with improvements in the computer system (factor of 2), and in off-the-shelf laser repetition rates (fictor of 5). Since there exists a trade-off between analysis time an sensitivity, any decrease in acquisition time will make the application of SALI mapping more practical. Instrumentation

A state-of-the-art SAL1 system combines both MPI and SPI capabilities. One commercial system3 includes two laser sources: a Nd-YAG laser with a gas tube assembly used for frequency tripling to produce the coherent 118-nm light for SPI; and an excimer laser that produces both 248-nm (KIF) and 193-nm (ArF) wavelengths used for MPI. The system also includes two ion-beam sources: a duoplasmatron (A+) or Cs+ion source, and a single or double lens liquid metal ion (Ga') source for SALI or TOF-SIMS mapping applications. Secondary Electron Detection (SED) images also can be obtained on this system, since it is equipped with an electron gun and the two ion guns. Each of these sources is compatible with the SED imaging system on the SALI instrument. The electron gun can also be used as an electronstimulated desorption source. The instrument includes a TOF reflecting mass analyzer, a low-energy electron flood source for charge neutralization, a sample introduction system, a sample manipulator and a UHV chamber. Conclusions

SALI is a relatively new surface technique that delivers a quantitative and sensitive measure of the chemical composition of solid surfices. Its major advantage, compared to its "parent" technique SIMS, is that quantitative elemental and molecular information can be obtained. SPI offers exciting possibilities for the analytical characterization of the surfaces of polymers and biomaterials in which chemical differentiation could be based solely on the characteristic SALI spectra. MPI is especiallyvaluable for elemental analyses with typical useful yield of 1O-3. Because SALI is laser-based, expected improvements over the next few years, in particular for vacuum-ultraviolet laser technology, should have a significant impact. High repetition rate Nd-YAG systems with sufficient pulse energy are already available to 50 Hz, and probably can be extended to a few hundred Hz.

568


Chapter 10

Ever brighter vacuum-ultraviolet sources are being developed that would hrther useful yield; general, sensitive boost SPI sensitivity, which already is typically elemental analysis would then also be available using SPI, making possible a single laser arrangement for both elemental and molecular SALI. Related Articles in the Encyclopedia

Static SIMS and SNMS References

C. H. Becker. In: Ion Spectroscopiesfir Suface Analysis. (A. W. Czanderna and D. M. Hercules, eds.) Plenum Press, New York, 1991, Chapter 4, p. 273. z D. G. Welkie, S. M. Daiser, and C. H. Becker. Vancum.41,1665, (1991); S.l? Mouncey, L.Moro, and C.H. Becker, Appl. Surf Anal. 52,39 (1991). 3 Perkin-Elmer Physical Electronics Division, Eden Prairie, MN, model 7

7000 SALI / TOF-SIMS instrument. Bibliography 1

W. Reuter, in Secondary Ion Mass Spectrometry SIMS Y Springer-Verlag, Berlin, 1986, p. 94.

A comparison of the various post-ionization techniques: electron-gas bombardment, resonant and nonresonant laser ionization, etc. While some of the numbers are outdated, the relative capabilities of these methods have remained the same. This is a well-written review article that reiterates the specific areas where post-ionization has advantages over SIMS. 2

3

J. B. Pallix, C. H. Becker, and N. Newman, MRS Bulletin, 12, no. 6,52 (1987). A discussion of the motivation behind doing sputtered neutral analysis versus SIMS, plus a description of the first prototype SALI instrument. A well written introduction for someone without previous surhce analysis experience it also includes an historical overview of the various post-ionization techniques. C. H. Becker, /. Irac. Sci. Technol,A51181 (1987). This article discusses why one would choose nonresonant multiphoton ionization for mass spectrometry of solid surfaces. Examples are given for depth profiling by this method along with thermal desorption studies.

10.3

SAL1

569

4

5

J. B. Pallix, C. H. Becker, and K. T. Gillen, Appl. Surface Sci, 32,l (1 988). An applications oriented discussion of using MPI-SAL1 for depth profiling, interface analysis in inorganic material systems. Examples of SAL1 depth profiles are given of a B implant in Si and the fluorine implanted electronic rest device which was referenced in this encyclopedia article. J. B. Pallix, U. Schiihle, C. H. Becker, and D. L. Huesris, Anal. G e m . , 61, 805 (1989). An introduction to the principles behind SPI-SALI, this article presents a theoretical discussion of why SPI-SAL1 is much less fragmenting than MPI-SALI. Examples are shown which describe the additional fragmentation induced by the desorption beam-in this case ESD is compared to ion sputtering. The main focus of the article is the advantages of SPI-SAL1 for surfice analysis of bulk organic polymers.

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Chapter 10

SNMS

10.4

Sputtered Neutral Mass Spectrometry J O H N C. H U N E K E

Contents Introduction Basic Principles SNMS Modes and Instrumentation Analysis and Quantitation Relative Sensitivity Factors Applications Conclusions

Introduction The atom flux sputtered from a solid surface under energetic ion bombardment provides a representative sampling of the solid. Sputtered neutral mass spectrometry has been developed as method to quantitativelymeasure the composition of this atom flux and thus the composition of the sputtered material. The measurement of ionized sputtered neutrals has been a significant improvement over the use of sputtered ions as a measure of flux composition (the process called SIMS), since sputter:d ion yields are seriously affected by matrix composition. Neutral particles are ionized by a separate process after sputter atomization, and SNMS quantitation is thus independent of the matrix. Also,since the sputtering and ionization processes are separate, an ionization process can be selected that provides relatively uniform yields for essentially all elements.

10.4

SNMS

57 1

I

SNMS ANALYSIS

I

PRIMARY (SPUTTERING) PARTICLES

Figure 1

Schematic of SNMS analysis. Neutral atoms and molecules sputtered from the sample surface by energetic ion bombardment are subsequently ionized for mass spectrometric analysis.

The erosion of the surfice by sputteringalso provides a means to sample progressively deeper layers to determine concentration depth profiles. SNMS combines the features of sputter erosion, representative sampling, uniform ionization and matrix independence to provide a quantitative sputtering depth profile measurement with comparable sensitivityfor all elements in complex thin-film structures comprisinga variety of matrix compositions. This capability is used to good advantage, for example, in the compositional analysis of thin-film structures used for magnetic recording heads, optoelectronics, and semiconductor metallizations. The detection limits obtained in depth profiling by SNMS range from 100 to 1000 ppm, which are not as low as can be generally obtained using SIMS, but which are a significant improvement over the detection limits obtained using AES and ESCA. Usehl overviews of all SNMS modes have been provided by Oeschner' and Pallix and Becker? and thorough reviews of electron impact SNMS in particular have been provided recently both by Ganschod and by Jede.* Basic Principles

The essentials of SNMS are illustrated in Figure 1. The surface of the solid sample is sputtered by energetic ion bombardment. Generally, at energies above a few hundred eV, several particles are ejected from the surface for each incident particle. A very small fraction of the particles are sputtered as ions, the so-called secondary ions 572


Chapter 10

measured by SIMS. By far the larger number of sputtered particles are neutral atoms and molecules, with atoms dominating. Coupling the nonselective sputter atomization with some method for nonselective ionization for mass spectrometric analysis, as illustrated in Figure 1,SNMS provides a technique in which the analpical signals directly reflect the sample’s composition, unlike SIMS where the ionization can be very selective. If the secondary ion component is indeed negligible, the measured SNMS ion currents will depend only on the ionizing mode, on the atomic properties of the sputtered atoms, and on the composition of the sputtered sample. Matrix characteristics will have no effect on the relative ion currents. SNMS analysis also provides essentially complete coverage, with almost all elements measured with equal facility. All elements in a chemically complex sample or thin-tilm structure will be measured, with no incompleteness due to insensitivity to an important constituent element. Properly implemented SNMS promises to be a near-universal analytical method for solids analysis. The SIMS analytical ion signal of a specific element or isotope also can be enhanced by selective ionization of particular atoms, and the detection limit for that element thereby improved. This mode of SNMS is important to specific applications, but it lacks the generality inherent in nonselective SNMS methods. The focus of this article will be on the methods for obtaining complete, accurate, and matrix-independent compositions of chemically complex thin-film strucrures and materials. SNMS Modes and Instrumentation Three post-ionization (i.e., post-sputtering) mechanisms having relatively uniform ion yields appropriate for SNMS have been used to date (cf. Figure l), and the combination of each of these with an appropriate mass spectrometer provides the basis for all present SNMS instruments. The posrioiization methods are: electron impact ionization, ionization by multiple photon interactions, and ionization by collision with metastable atoms in a plasma. The SNMS methods incorporating the latter two modes are nonresonant multiphoton ionization mass spectrometry, which is also referred to as Surface Analysis by Laser Ionization (SALI), and GlowDischarge Mass Spectrometry (GDMS). Both are the subject of other articles in this encyclopedia but are mentioned here because of analytical affinities. This micle will concentrate on the first mode, SNMS by electron impact ionization. The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electrongun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 8010.4

SNMS

573

a. SIMS MS

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0 0 0

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c

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Relationship of SIMS, separate bombardment SNMSs and direct bombardment SNMSd. (a1 Materials for SIMS analysis are those ions formed in the sputtering with a focused primary ion beam. The largest fraction of the particles sputtered from the surface are neutral atoms. (b) Ions for SNMS analysis are formed by ionization of the sputtered neutrals. (GI When the plasma is used as an ionizer, plasma ions can also be used t o sputter the sample surface at low energies.

100 eV energy in the ionizer volume. Sputtering of the sample surface in elearongun SNMS is accomplished by a focused ion beam, as provided for the SIMS instrumentation (Figures 2a and 2b). An electron-gun ionizer is commonly used to modify existing SIMS instrumentation to provide supplementary SNMS capability, and SNMS measurements using the separate ion-gun sputtering provided for SIMS will have the same imaging and thin-film depth resolution capabilities as for SIMS. In contrast to SIMS, a noble gas (e.g., Ar) or less reactive element (such as Ga) is most commonly used for ion-beam sputtering in SNMS to minimize 574


Chapter 10

enhancement of secondary ion yields and thus improve quantitative accuracy. Following the initial development and use of electron-gun SNMS in the academic research environment, ONO such SNMS modules have been made commercially available. In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufficientlydense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in AI, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electrongas SNMS is still in the developmental stages, while the former has been incorporated into commercial instrumentation. The plasma electron-gas ionizer offers a distinct advantage for low-energy sputtering of the sample surface, since Ar ions are available in the overlying plasma and can be accelerated onto the surface by application of a sample bias voltage (Figure 2c). In neither of the electron-gas SNMS modes does the sample play an integral part in maintaining the plasma. Thus the bias voltage can be made arbitrarily small, enabling plasma-ion sputtering with minimal energy, ion-beam mixing, and thus optimal depth resolution (see below). The surface areas analyzed in the plasma mode of surface sputtering are large compared to SIMS or SNMS profiling by separate focused ion-beam bombardment, and lateral resolution is sacrificed. The SNMS instrumentation that has been most extensively applied and evaluated has been of the electron-gas type, combining ion bombardment by a separate ion beam and by direct plasma-ion bombardment, coupled with postionization by a low-pressure RF plasma. The direct bombardment electron-gas SNMS (or SNMSd) adds a distinctly different capability to the arsenal of thin-film analytical techniques, providing not only matrix-independent quantitation, but also the excellent depth resolution available from low-energy sputtering. It is from the application of SNMSd that most of the illustrations below are selected. Little is lost in this restriction, since applications of SNMS using the separate bombardment option have been very limited to date. Analysis and Quantitation

In the process of SNMS analysis, sputtered atoms are ionized while passing through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surhce is sputtered away. 10.4

SNMS

575

Only the knowledge of relative useful ion yields and isotopic abundances is required to calculate elemental composition from the relative ion current measurements. The usefd ion yield 0,is the number of ions x+ detected relative to the number of atoms of element xsputtered. The measured relative ion current of two isotopes is

2 (;)(:)g) S

=

where c, is the concentration and 4 is the isotopic fraction of the measured isotope of element x. Pragmatically, quantitation is accomplished by multiplying the ratios of the total ion currents for each element (summingover all isotopes of the element) by a multiplicative factor defined as the relative sensitivity factor or RSF.

The fraction of each element present in the material is then equal to the ratio of the RSF-corrected ion current for that element to the sum of the RSF-corrected ion currents for all elements. It is important for quantitative SNMS that the fractions of element x forming molecules and sputtered ions be negligible, but such is not always the case. Relative Sensitivity Factors

The relative sensitivity factors for most elements are comparable to within a factor of 25 for ionization with an energetic electron gas.5 The RSFs for a number of elements determined from the analysis of NIST alloy samples vary by less than an order of magnitude for sputtering energies of 1250 V and more. The RSFs determined are reasonably independent of matrix. Nevertheless, there are differences of up to factors of 1.5 in RSFs of the same element determined from the analysis of several standards. Also, RSFs do change significantly with sputtering voltage. As a consequence, separate calibration is required when sputtering at the lower energies typical of depth profiling. Similar detailed studies of RSFs have been carried out for GDMS, but not for electron-gun electron impact ionization or for SALI. The spread in elemental RSFs for electron-gas SNMS is comparable to that observed for Ar glow-discharge ionization of sputtered neutrals.6 Since elemental RSFs are reasonably similar for electron-gas SNMSd, a standardless analysis will result in compositions accurate to within a factor of 5 for matrices with major element RSFs dose to the average, and to within a factor of 25 for matrices with major element RSFs at the extreme values. More importantly, 576


Chapter 10

104 103

10’ IO’

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3

105

0

10‘

z 0

I03

IO‘ IO‘

140

160

2co

180

220

240

MASS Figure 3

Mass spectrum obtained from the NIST Hasteloy Ni-based standard alloy, using electron-gas SNMSd (Leybold INA-3). The sputtering energy was 1250 V, increasingthe sputtered atom flux at the expense of depth resolution. Matrix ion currents were about 105 cps, yielding background limited detection at about 2 ppm.

however, there will be no glaring gaps in the analytical results due to extreme insensitivity for a particular element. Every element present will be detected at roughly the same sensitivity. This characteristic of SNMS enables thorough materials characterization of complex samples in a single analysis and by one instrument. Applications Bulk Analysis

Independent of depth profiling considerations, SNMS provides a powerful bulk analysis method that is sensitive and accurate for all elements, from major to trace element levels. Since SNMS is universally sensitive, it offers obvious advantages over elementally selective optical methods. As an example of a standardless bulk analysis by SNMS, a measurement of the complex Ni-based Hasteloy metal (NIST SRM 2402) is presented in Figure 3 and Table 1, in which the “composition”determined from ion-current ratios (not RSF corrected) is compared to the certified chemical composition. It is very evident in Figure 3 that the chemical complexity of Hasteloy presents special problems for mass spectrometric analysis using a quadrupole mass spectrometer with low mass resolution. Molecular ions comprised of combinations of matrix and plasma atoms are formed in abundance and will obscure many elements 10.4

SNMS

577

Hement

Ion Current ratio (%)

Certified content (% at.)

C

0.05

Si

1.90

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0.005

0.014

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0.02

0.035

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0.6

0.27

Cr

26

19.5

Mn

0.6

0.73

Fe

9

8.2

co

2

1.6

Ni

37

cu Mo

w Table 1

0.07 25

0.9

55 0.19 11.2 1.46

Semiquantitative bulk analysis by SNMSd of the NIST SRM 2402 Hasteloy metal standard.

present at low concentrations. This is obviously a clear limitation to many applications, and the use of a high resolving power mass spectrometer is to be preferred in these instances. Molecular ion mass interferences are not as prevalent for the simpler matrices, as is dear from the mass spectrum obtained for the Pechiney 11630 AI standard sample by electron-gasSNMSd (Figure 4).For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfactory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. Quantitative Depth Profiling

In addition to comprehensive elemental coverage, SNMS also provides for highresolution depth profile measurements with the same quantitation capability 578


Chapter 10

10.

n

cn

K9

‘0

io*

Y

10;

2

85

106

5

105 10. IO’ 102

lo$ 120

140

160

200

MASS Figure 4

Mass spectrum obtained from the Aluminium Pechiney standard AI 11630, using electrongasSNMSd with a sputtering energy of 1250 V. The nAl matrix ion current was significantly greater than 10’ cps, yielding a background count rate limit less than 1 ppm.

throughout the depth profile regardless of film composition. This feature of SNMS is particularly u s e l l for the measurement of elements located in and near interfaces, which are difficult regions for measurement by other thin-film analytical methods. The advantage of SNMSd for high-resolution profiling derives from the sputtering of the sample surface at arbitrarily low energies, so that ion-beam mixing can be reduced and depth resolution enhanced. Excellent depth resolution by SNMSd depth profiling is well illustrated by the SNMSd depth profile of a laser diode test structure shown in Figure 5. Structures of this type are important in the manufacture of optoelectronics devices. The test structure is comprised of a GaAs cap overlying a sandwiched sequence of AlxGal-& layers, where the intermediate Al-poor layer is on the order of 100 thick. The nominal compositions from growth parameters are noted in Figure 5. The layers are very well resolved to about a 30-A depth resolution, with accurate composition measurement of each individual layer. Every material sputters at a characteristic rate, which can lead to significant ambiguity in the presentation of depth profile measurements by sputtering. Before an accurate profile can be provided, the relative sputtering rates of the components of a material must be independently known and included, wen though the total depth of the profile is normally determined (e.g., by stylus profdometer). To first order, SNMS offers a solution to this ambiguity, since a measure of the total number of atoms being sputtered from the surface is provided by summing all RSF- and

a

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SNMS

579

' I 50

Figure 5

100

150

200

250

Quantitative high depth resolution profile of a complex AI,Ga,,As laser diode test structure obtained using electron-gas SNMS in the direct bornbardrnent mode, with 6 0 0 4 sputtering energy. The data have been correqed for relative ion yield variations and summed to AI + Ga = 50%. The 100-A thick GaAs layer is very well resolved.

isotope-corrected ion currents (assuming all major species have been identified and included in the measurements.) It is necessary only to scale the time required to profile through a layer by the total sputtered neutral current (allowing for atomic density variations) to have a measure of the relative layer thickness. The profiles illustrated in Figure 5 have not been corrected for this effect. AI Metallization

The measurement of the concentration depth profiles of the minor alloying elements Si and Al in Al metallizations is also very important to semiconductor device manufacturing. The inclusion of Si prevents unwanted alloying of underlying Si into the Al. The Cu is included to prevent electromigration. These alloying elements are typically present at levels of 1% or less in the film, and the required accuracy of the measurement is several percent. Of the techniques that can be applied to this analysis, SNMS offers the combined advantages of sensitivity to both Si and Cu, good detection limits in the depth profiling (0.01-0.1%), and accuracy of analysis, as well as requiring measuring times on the order of only one-half hour. 580


Chapter 10

DEPTH (urn) Quantitative depth profile of the minor alloying elements Cu and Si in AI metallization on SiO,/Si, using electron-gas SNMSd.

Figure 6

A typical SNMSd profile ofAl(1% Si, 0.5% Cu) metallization on Si02 is shown in Figure 6. The 0 signal is included as a marker for the Al/SiOZ interface. The Al mamix signal is some lo5 cps, yielding an ion count rate detection limit of 10 ppm for elements with similar RSF. The detection limit is degraded from this value by a general mass-independent background of 5 cps and by contamination by 0 and Si in the plasma. It does not help that in this instance the product (ion yield) x (isotopic abundance) for Cu is an order of magnitude lower than for Al. Nonetheless, the signals of both Si and Cu are quite adequate to the measurement. The Si exhibits a strongly varying composition with depth into the film, in contrast to the Cu distribution. Diffusion Barriers

An important component of the complex metallizations for both semiconductor devices and magnetic media is the diffusion barrier, which is included to prevent interdiffusion between layers or diffusion from overlyinglayers into the substrate. A good example is placement of a TiN barrier under an Al metallization. Figure 7a illustrates the results of an SNMSd high-resolution depth profile measurement of a TiN diffusion barrier inserted between the Al metallization and the Si substrate. The profile dearly exhibits an uneven distribution of Si in the Al metallization and has provided a clear, accurate measurement of the composition of the underlying TiN layer. Both measurements are difficult to accomplish by other means and dem10.4

SNMS

581

0.2

0.4

0.6

DEPTH Cum)

b 4.

2. n

ae

Y

Z

Ea

E

10' 6. 4.

2. 100 6. 4.

W

2.

0 10-1

6. 4.

2.

0.2

0.4

0.6

0.e

DEPTH bm)

Figure 7

582

Quantitative high depth resolution profile of the major elements in the thinfilm structure of AI /TiN /Si, comparing the annealed and unannealed structures t o determine the extent of interdiffusion of the layers. The depth profile of the unannealed sample shows excellent depth resolution (a). The small amount of Si in the AI is segregated toward the A I / l i N interface. After annealing, significant Ti has diffused into the AI layer and AI into the TIN layer, but essentially no AI has diffused into the Si (b). The Si has become very strongly localized at the AI /TIN interface.


Chapter 10

100

200

300

400

500

600

TIME (SI Figure 8

Quantitative high depth resolution profile of 0 and N in a Ti metal film on Si, using electron-gasSNMS in the direct bombardmentmode. Both 0 and N are measured with reasonably good sensitivity and with good accuracy both at the heavily oxidized surface and at the Ti/Si interface.

onstrate the strength of SNMS for providing quantitative measurements in all components of a complex thin-film structure. The results of processing this structure of Al:Si/TiN/Si are shown in Figure 7b. The measurement identifies the redistribution of the Si to the interfaces, the diffusion of Al and Si into the TiN, and a strong difiiiion out of Ti from the TiN into the overlyingAl. However, no Al has diffused into the Si nor Si from the substrate into the Al, demonstrating the effectivenessof the TiN barrier. Yet another strength of SNMS is the ability to measure elemental concentrations accurately at interfaces, as illustrated in Figure 8, which shows the results of the measurement of N and 0 in a Ti thin film on Si. A substantial oxide film has formed on the exposed Ti surface. The interior of the Ti film is free of N and 0,but significant amounts of both are observed at the Ti/Si interface. SNMS is as sensitive to 0 as to N, and both the 0 and N contents are quantitativelymeasured in all regions of the structure, including the interface regions. Quantitation at the interface transition between two matrix types is difficult for SIMS due to the matrix dependence of ion yields. 10.4

SNMS

583

Conclusions The combination of sputter sampling and postsputtering ionization allows the atomization and ionization processes to be separated, eliminating matrix effects on elemental sensitivity and allowing the independent selection of an ionization process with uniform yields for essentially all elements. The coupling of such a uniform ionization method with the representativesampling by sputteringthus gives a “universal’’ method for solids analysis. Electron impact SNMS has been combined most usefully with controlled surface sputtering to obtain accurate compositional depth profiles into surfaces and through thin-film structures, as for SIMS. In contrast to SIMS, however, SNMS provides accurate quantitation throughout the analyzed structure regardless of the chemical complexity, since elemental sensitivity is matrix independent. When sputtering with a separate focused ion beam, both image and depth resolutions obtained are similar to the those obtained by SIMS. However, using electron-gas SNMS, in which the surfice can be sputtered by plasma ions at arbitrarily low bombarding energies, depth resolutions as low as 2 nrn can be achieved, although lateral image resolution is sacrificed. In summary, the forte of SNMS is the measurement of accurate compositional depth profiles with high depth resolution through chemically complex thin-film structures. Current examples of systems amenable to SNMS are complex III-IV laser diode structures, semiconductor device metallizations, and magnetic readwrite devices, as well as storage media. SNMS is still gaining industrial acceptance as an analytical tool, as more instruments become available and an appreciation of the unique analytical capabilities is developed. To date, SNMS has not become established as a routine analytical tool providing essential measurements to a significant segment of industry. The technique still remains largely in the domain of academic and research laboratories, where the 111 range of application is still being explored. The present stage of SNMS development is appropriate to this environment, and refinements in hardware and s o b a r e can be expected, given a unique niche and the pressure of commercial or industrial use. In addition to the analysis of complex thin-film structures typical of the semiconductor industry, for which several excellent examples have been provided, an application area that offers hrther promise for increased SNMS utilization is the accurate characterization of surfkces chemically modified in the outer several hundred-A layers. Examples are surfaces altered in some way by ambient environments-a sheet steel surface intentionally altered to enhance paint bonding, or phosphor particles with surfaces altered to enhance fluorescence. A strength of SNMS that will also become more appreciated with time is its ability to provide, with good depth resolution, quantitative measurements of material trapped at interfaces, for example, contaminants underlying deposited thin films or migrating to interfacial regions during subsequent processing. As these and other application 584


Chapter 10

areas are explored more fully, the place of SNMS will become more evident and secure, and the evolution of SNMS instrumentation even more rapid. Related Articles in the Encyclopedia

SIMS, SALJ, and LIMS References

H. Oechsner. ScanningMimscopy. 2,9,1988. z J. B. Pallix and C. H. Becker. In Advunced Cbaractehtion Z c b n i p e s j r Ceramics.(G. L. McVay, G. E. Pi,and W. S. Young, Fds.)ACS, W a e r I

ville, 1989. 3

4

5 8

0. Ganschow. In Analytical Zcbniqmjr SemicondzrctorMatn;ialjand Process CburacteriMtion. The Electrochemical Society, Pennington, 1990, VOL90-1 1, p. 190. R Jede. In Secondary Ion Mass Spectromeny. (A. Benninghoven, C. -A. Evans,K. D. McKeegan, H. A. Storms, and H. W. Werner, Eds.) J. W i l e y and Sons, New York, 1989, p. 169. A. Wucher, E No&, and W. Reuter. J k. Sei. Technol. A6,2265,1988. W. Vieth and J. C. Huneke. Specmcbim. Acta 46B (2), 137,1991.

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SNMS

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10.5

LIMS Laser Ionization Mass Spectrometry F I L I P P O RADICATI D I B R O Z O L O

Contents

Introduction Basic Principles Instrumentation Applications Sample Requirements Conclusions

Introduction

Laser ionization mass spectrometryor laser microprobing (LIMS) is a microanalyti d technique used to rapidly characterizethe elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses 10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are dose to 10l6 at/cm3, which makes this technique considerablymore sensitivethan other survey microanalytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA).Additionally, LIMS can be used to analyze insulating sam( 4

586


Chapter 10

ples, as well as samples of complex geometry. Another advantage of this technique is its ability sometimes to provide basic molecular information about inorganic as well as organic surface contaminants. A growing field of application is the characterization of organic polymers, and computerized pattern recognition techniques have been successfully applied to the classification of various types of mass spectra acquired from organic polymers. The LIMS technique is rarely used for quantitative elemental analysis, since other techniques such as EPMA, AFS or SIMS are usually more accurate. The limitations of LIMS in this respect can be ascribed to the lack of a generally valid model to describe ion production from solids under very brief laser irradiation. Dynamic range limitations in the LIMS detection systems are also present, and will be discussed below. Basic Principles

LIMS uses a finely focused ultraviolet (w)laser pulse (210 ns) to vaporize and ionize a microvolume of material. The ions produced by the laser pulse are accelerated into a time-of-flight mass spectrometer, where they are analyzed according to mass and signal intensity. Each laser shot produces a complete mass spectrum, typically covering the range 0-250 m u . The interaction of laser radiation with solid matter depends significantly upon the duration of the pulse and the power density levels achieved during the pulse.' When the energy radiated into the material significantly exceeds its heat of vaporization, a plasma (ionized vapor) cloud forms above the region of impact. The interaction of the laser light with the plasma cloud further enhances the transfer of energy to the sample material. As a consequence, various types of ions are formed from the irradiated area, mainly through a process called nonresonant multiphoton ionization (NRMPI). The relative abundances of the ions are a function of the laser's power density and the optical properties and chemical state of the material. Typically, the ion species observed in LIMS include singly charged elemental ions, elemental cluster ions (for example, the abundant C y negative ions observed in the analysis of organic substances), and organic fragment ions. Multiply charged ions are rarely observed, which sets an approximate upper limit on the energy that is effectively transferred to the material.', The material evaporated by the laser pulse is representative of the composition of the solid,' however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser .irradiance conditions (>IO'' W/cm2) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100-200 ppm atomic for elements with poor ion yields (for example, Zn or As). 10.5

LIMS

587

Figure 1

Schematic diagram of a LIMS instrument, the LIMA 2A (Cambridge Mass Spectrometry, Ltd., Kratos Analytical, UK).

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (h= 266 nrn) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efficiency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analyti d application is discussed below. Instrumentation

The schematic diagram of a LIMS instrument is shown in Figure 1. The instrument's basic components include: 'I A Qswitched, frequency-quadrupledNd-YAG laser (h= 266 nm) and its accompanying o p t i d componenrs produce and focus the laser pulse onto the sample surfice. The typical laser spot size in this instrument is approximately 2 pm. A He-Ne pilot laser, coaxial with the U V laser, enables the desired area to be located. A calibrated photodiode for the measurement of laser energy levels is also present 588


Chapter 10

I O Y l l l Y O LAUER

Schematic view of the ion source region of the LlMS instrument in the PA1 configuration.

Figure 2

2

The time-of-flight mass spectrometer (under high vacuum) consists of a sample stage equipped with y z motion, the ion extraction region, and the ion flight tube (approximately 2 m in length) with energy focusing capabilities

3

The ion detection system consists of a high-gain electron multiplier and the signal digitizing system, along with a computer for data acquisition and manipulation.

Figure 2 presents a schematic view of the ion source region in the PAI configuration. A second high-irradiance, frequency quadrupled pulsed Nd-YAG laser is focused parallel to and above the sample surface, where it intercepts the plume of neutral species that are produced by the ablating laser. Appropriate focusing optics and pulse time-delay circuitry are used in this configuration. A typical LIMS analysis is performed by positioning the region of interest of the sample by means of the He-Ne laser beam, after which the Nd-YAG laser is fired. The W laser pulse produces a burst of ions of different masses from the analytical crater. These ions are accelerated to almost constant kinetic energy and are injected into the spectrometer flight tube. As the ions travel through the flight tube and through the energy-focusing region, small differences in kinetic energy among ions of the same mass are compensated. Discrete packets of ions arrive at the detector and give rise to amplified voltage signals that are input to the transient recorder. The function of the transient recorder is to digitize the analog signal from the electron multiplier, providing a record of both the arrival time and the intensity of the signals associated with each mass. The data are then transferred to the computer for further manipulation, the transient recorder is cleared and rearmed, and the instrument is ready for the acquisition of another spectrum. 10.5

LlMS

589

This sequenceof events is quite rapid. If we take typical instrumental conditions

of the LIMA 2A, where the UV laser pulse duration is 5-10 ns, the fight path is -2 m, and the accelerating potential is 3 kV, then an H+ion arrives at the detector i n approximately 3 ps, and a U+ion arrives at the detector in approximately40 p. Since the time width of an individual signal can be as short as several tens of nanoseconds, a high speed detection and digitizing system must be employed. Typical mass resolution values measured on the LIMA 2A range from 250 to 750 at a mass-to-charge ratio M/Z= 100. The parameter that appears to have the most influence on the measured mass resolving power is the duration of the ionization event, which may be longer than the duration of the laser pulse (5-10 ns), along with probable time broadening effects associated with the 16-11s time resolution of the transient re~order.~ The intensity of an ion signal recorded by the transient recorder is proportional to the number of detected ions. There are two limiting factors to this proportionality, one due to the nonlinear output of the electron multiplier at high-input ion signals, and the other due to the dynamic range of the digitizer. The dynamic range of typical venetian blind-type electron multipliers for linear response to fast transients is less than four orders of magnitude. Electron multipliers characterized by other geometries (mesh type) are currently being evaluated, and may provide a larger inherent dynamic range.3 The second limiting factor in the quantitative measurement of the ion signal intensity is associated with the digitization of the electron multiplier output signal by the transient recorder. For example, the Sony-Tektronix 390 AD transient recorder in the LIMA 2A is a 10-bit digitizer with an effective dynamic range of 6.5 bits for 10-MHzsignals. This device provides approximately90 discretevoltage output levels at input frequencies oftypical ion signals.* Limitations in the digitizer’s dynamic range can be overcome by using multiple transient recorders operating ar difirent sensitivities, or by adding logarithmic preamplifiers in the detection system. From the preceding discussion it appears, however, that quantitative analysis is not the primary area of application of LIMS. Semiquantitative and qualitative applications of LIMS have been developed and are discussed in the remainder of this article. Applications

Most applications of LIMS are in failure analysis. A typical microanalytical failure analysis problem, for example, may involve determinating the cause of corrosion in a metallization line of an integrated circuit. One can achieve t h i s by perfbrming an elemental survey analysis of the corroded region. Since it is not always known which elements are normal constituents of the material in question and which are truly contaminants, the vast majority of these analyses are performed by comparing the elemental make-up of the defective region to that of a control region. The com590


Chapter 10

Positive-ion mass spectrum acquired from defective sample. Intense copper ion signals are observed W / Z = 63 and 65).

Figure 3

parison of mass spectra of the two regions may reveal the presence of additional elements in the defective region. Those elements are often the cause or byproducts of the corrosion. In this type of analysis, the selection of a relevant control sample is obviously critical.' LIMS analytical applications may be classified as elemental or molecular survey analyses. The former can be further subdivided into surface or bulk analyses, while molecular analyses are generally applicable only to surface contamination. In the following descriptions of applications, a comparison with other analytical techniques is presented, along with a discussion of their relative merits. Bulk Analysis

One example of the application of LIMS to bulk contamination microanalysis is the analysis of low level contamination in GaP light emitting diodes (LEDs). The light emission characteristics of GaP LEDs can be severely affected by the presence of relatively low levels of transition elements. Although the nature of the poisoning species may be suspected or inferred from intentional contamination experiments, the determination of elemental contaminants in actual failures is a difficult analytical problem, in particular because of the small size and complex geometry of the parts. Figures 3 and 4 illustrate two positive-ion mass spectra that were acquired from cross sections of a defective and a nondefective GaP LED, respectively. The laser power density employed in this analysis was high to maximize the detection of low-level contaminants. The depth of sampling is estimated to be 1000-1500 A. The two mass spectra exhibit intense signals for Ga+,along with moderately intense signals for I?+. The defective LED also exhibits readily recognizable signals at M / ' Z = 63 and 65, matching in relative intensity the two Cu isotopes. The presence of Cu in the defective LED can explain its anomalous optical behavior. This 10.5

LIMS

591

ION UASS l m / d

Figure 4

Positive-ion mass spectrum acquired from the contact region of a control sample. Copper ion signals are absent.

example is a good illustration of the unique advantages of LIMS over other analytical techniques. These include the ability to perform rapid survey analysis to detect unknown contaminants. Two other advantages of LIMS illustrated by this example are its ability to analyze a small sample having nonplanar geometry, without timeconsuming sample preparation, and its sensitivity,which is superior to that of most electron beam techniques. Surface Analysis

An example of elemental contamination surface microanalysis is shown inFigure 5. This is a negative-ion mass spectrum acquired from a small window (-4 pm) etched through a photoresist layer deposited onto a HgCdTe substrate. An AI film is then deposited in these windows to provide electrical contact with the substrate. Windows were found to be defective because of poor adhesion of the metallic layer. The spectrum shown in Figure 5 was acquired from a defective window, and reveals the presence of intense signals of C1- and Br-, neither of which is observed in similar regions with good adhesion characteristics. In t h i s case, the photoresist had been etched with solutions containing Cl and Br. The laser power density employed in this analysis was low, and the sampling depth was estimated to be e 500 8. This analysis indicates that poor adhesion on the contaminated windows is due to incomplete rinsing of etching solutions. The ability of LIMS to operate on nonconductive materials is a major advantage in this case, since both the HgCdTe substrate and the surrounding photoresist are insulating. Techniques that use charged-partid e beams (electrons,AES or EPMA; ions, SIMS) could probably not be applied in th'IS case. 592


Chapter 10

T.

0

20

40

80

M)

1W

120

140

rn,C

Diagram of the windows cut in the photoresist on an H g m e substrate (top). Spectrum acquired from a defective window, and reveals the presence of intense signals of Cl-and Br- (bottom).

Figure 5

Organic Surface Microanalysis

The laser irradiation of a material can produce molecular ions, in addition to elemental ions, if the power density of each pulse is sufficiently low. The analysis of such molecular species includes the study of organic materials ranging from polymers to biological specimens, as well as the analysis of known or suspected organic surface contaminants. LIMS organic spectroscopy is primarily a qualitative technique, which is used to identify a number of fragment ions in a spectrum that are diagnostic for a given class of organic species. Organic contamination in the microelectronics industry is often related to the presence of organic polymer residues, for example, photoresist. These organic residues are a serious problem for surface adhesion, for example in the case of bond pads. Examples of LIMS organic surface analysis are shown in Figures 6 and 7: mass spectra acquired from a commercial photoresist, using positive- and negative-ion detection, respectively. The positive-ion mass spectrum in Figure 6 exhibits intense signals for alkalis (Na and K) along with a series of signals that are C-based fragment ion peaks. Some of these are undoubtedly aromatic fragment ions ( M / Z = 77, 91, and 1 15, among others), and are diagnostic of this particular photoresist. Similarly, the negative-ion mass spectrum in Figure 7 exhibits an intense signal at M / 2 = 107, which arises from Novolak resin, one of the constituents of the photoresist. Other, less intense signals of this spectrum include the species SO2, SO,, and HSO,, which are also known to be present in the photoresist. Laser Post-Ionization of Ablated Neutrals

A ZnSe-on-GaAs epitaxial layer required a sensitive survey of near-surface mntamination. PAI was selected for ZnSe analysis because its major constituents and many of the expected impurities are elements that have poor ion yields in conventional LIMS. Figures 8 and 9 are two mass spectra acquired from the ZnSe epitaxial layer, 10.5

LIMS

593

w m 6 t 1

El,[

Figures 0 and 7 Mass spectra acquired from a commercial photoresist, using positive- and negative-ion detection, respectively.

using conventional single-laser LIMS and the PAI configuration, respectively. The single-laser spectrum in Figure 8 exhibits primarily the Zn' and Se+signals, and weak signals for Cr' and Fe'. The high background signal level following the intense Se signal is related to detector saturation. The ablator laser irradiance for this spectrum was estimated to be > 10" W/cm2, hence the high background signal. In contrast, the PAI mass spectrum in Figure 9 exhibits readily observable signals for Cd+and Te+in addition to the Zn and Se signals. Note also the low background in the region that follows the Se signal. The ablator laser irradiance in the PAI spectrum was approximately lo9 W/cm2, a factor of 10 lower than in the single-laser analysis. The lower ablator laser irradiance samples the top 100 A of the sample, compared to 1000 A or more in single-laser analysis, and hence provides better sur594


Chapter 10

13=

>

55

132

I-

z

a '

s

10'

c

50

100

150

200

25c

ION MASS h/z)

Figure 8

Conventional, single-laser mass spectrum of ZnSe.

Figure 9

Spectrum of ZnSe using the two-laser (PA11instrumental configuration.

face sensitivity. In conclusion, the PAI variant of LIMS is especiallyuseful when the elements present have high ionization potentials that preclude efficient ion detection via conventional LIMS analysis, and in those cases when a higher surfice sensitivity is desired. Sample Requirements

A general requirement for LIMS analysis is that the material must be vacuum compatible and able to absorb W laser radiation. With regard to the latter require10.5

LIMS

595

ment, the absorption characteristics of UV-transparent materials can be improved with the use of thin UV-opaque coatings, such as Au or C. Care must be exercised, however, that the coating does not introduce excessive contamination, and practice is needed to determine the best coating for each sample. A typical LIMS instrument accepts specimens up to 19 mm (0.75 in) in diameter and up to 6 mm in thickness. Custom designed instruments exist, with sample manipulation systems that accept much larger samples, up to a 6-in wafer. Although a flat sample is preferable and is easier to observe with the instrument's optical system, irregular samples are often analyzed. This is possible because ions are produced and extracted from pm-sized regions of the sample, without much influence from nearby topography. However, excessive sample relief is likely to result in reduced ion signal intensity. The electrical conductivity of the sample is, to a first approximation, much less critical than in the case of charged-particle beam techniques (e.g., A E S or SIMS), because the laser beam does not carry an electric charge, and is pulsed with a very low duty cycle. However, charging effects are sometimes observed in the negativeion analysis of insulating samples, such as ceramics or silicon oxide. Charging probably arises from the acceleration of large numbers of electrons from the sample surface, along wirh the negative ions, which leaves behind a positively charged sample surface. Effects of this type may be alleviated with the use of conductive masks over the sample surface. Conclusions

LIMS is primarily used in failure microanalysis applications, which make use of its survey capability, and its high sensitivity toward essentially all elements in the periodic table. The ability to provide organic molecular information on a microanalytical scale is another distinctive feature of LIMS, one that is likely to become more important in the future, with improved knowledge of laser desorption and ionization mechanisms. Future trends for LIMS are likely to include hardware improvements, theoretical advances in the understanding of the basic mechanisms of laser-solid interactions, and improved methods for data handling and statistical analysis. Among the hardware improvements, one can count the advent of post-ionization techniques, which are briefly presented in this article and are discussed elsewhere in the Encyclopedia, and improvements in detection system dynamic ranges, through the use of different types of electron multipliers and improved transient recorders. These innovations are expected to result in improved quantification of the results. The introduction of faster pulsed lasers may also prove a significant improvement in mass resolution for LIMS, thus making it more suitable for organic analysis. Improvements in software may include compilations of computerized databases of LIMS organic mass spectra, the development of pattern recognition techniques, and the introduction of expert systems in the analysisof large bodies of LIMS data.' 596


Chapter 10


SALI, SIMS, SNMS, GDMS, and A E S References 1

I. D. Kovalev et al. Int. J. Mass Spectrom. Ion Phys. 27, 101, 1978. Contains a discussion of laser-solid interactions and ion production under a variety of irradiation conditions.

u.

2

T. Dingle and B. W. Griffiths.In Microbeam Analysir-1985 T. Armstrong, ed.) San Francisco Press, San Francisco, 315, 1985. Contains examples of quantitative analytical applications of LIMS.

3

R W. Odom and B. Schueler. Laser Microprobe Mass Spectrometry: Ion and Neutral Analysis. in h e r s and Mass Spectrometry (D. M. Lubman, ed.) Oxford University Press, Oxford, 1990. Presents a useful discussion of LIMS instrumental issues, including the post-ablation ionization technique. Several analytical applications are presented.

4

D. S. Simons. Int. J. Mass Spectrom. Ion Process. 55,15, 1983. General discussion of the LIMS technique and its applications. Contains a discussion of detector dynamic range issues.

5

L. Van Vaeck and R Gijbels. in Microbeam Analysis-1989 (I?E. Russell, ed.) San Francisco Press, San Francisco, xvii, 1989. A synopsis of laserbased mass spectrometry analytical techniques.

6

I!B. Harrington, K. J. Voorhees, T. E. Street, E Radicati di Brozolo, and R W. Odom. Anal. Chm. 61,715,1989. Presents a discussion of LIMS polymer analysis and pattern recognition techniques.

10.5

LIMS

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10.6

SSMS Spark Source Mass Spectrometry WILLIAM L. H A R R I N G T O N

Contents

Introduction Basic Principles Comparison With Other Techniques Conclusions

Introduction

No single trace elemental technique can provide a complete analysis of the many materials used in today’s high technology applications. In the 30 years since its commercial introduction, SSMS has proven to be a versatile technique that can be applied to a wide variety of material types. The high-voltage radiofrequency (RF) spark source, which is used to volatilize and ionize the sample elements, has been shown to do so with relatively uniform probability across the entire periodic table. Although far from perfect in this respect, sensitivities for most elements in most matrices are therefore uniform and are constant within about a factor of 3, even at trace levels of less than 1 part per million atomic (ppma). Like most mass spectrometric techniques, SSMS is linear with respect to concentration over a wide range, achieving 8-9 orders of magnitude in cases where the signals are interference free. This relatively uniform, high sensitivity, combined with the ability to examine materials in a wide variety of forms, makes SSMS an excellent choice for trace elemental surveys of bulk and some thin-film specimens. The three most-used trace element techniques for survey analysis are Emission Spectrometry (ES), Glow Discharge Mass Spectrometry (GDMS), and SSMS. After a detailed discussion of SSMS, a comparison with these and other techniques will be made. 598


Chapter 10

Bulk solids ~~

~

~

Silicon (boules and wafers)

GaAs Evaporation sources (e.g., Al, Au, and Ti) Precious metals (Pt, Au, Rh wire, and melts) Steel Alloys (Ni-Co-Cr, AI-Si, Cu-Ni, and Inconel) Ga metal (cooled to solid phase using liquid nitrogen) Powders Graphite Rare earth oxides and phosphors Ceramics (A1203) and glasses Mining ores and rock Superconductors and precursor materials Thin films

Silicon wafers and Si02 films

Si / sapphire (SOS), Si / insulators (SOI) Epiraxial GaAs Buried oxides (SIMOX) Plated, sputtered, or evaporated metals Table 1

Typical materials analyzed.

Table 1 lists some of the materials typically analyzed by SSMS and some of the forms in which these materials may exist. T h e basic requirement is that two conducting electrodes be formed of the material to be analyzed. Details of the analysis of each type of sample will be discussed in a later section. Although a number of studies have been made concerning the basic properties of the RF vacuum spark used for excitation,14 the discharge is typically erratic, producing a widely fluctuating signal for mass analysis. For this reason, the most widely used form of this instrumentation consists of a mass spectrometer of the 10.6

SSMS

599

PULSED DISCHARGE CIRCUIT

SAMPLE

1ooKvMAx.

BEAM SUPPRESSOR ELECTRODE

SAMPLE

I;- Ii(m)+ID 2+(m)+...

SAMPLE' ELECTRODE

+20kV

I MATRIX-m IMPURITY x

-

MAGNET

Figure 1

Schematic diagram of a Mattauch-Herzog geometry spark source mass spectrometer using an ion-sensitive plate detector.

Mattauch-Herzog g e ~ m e t r y ,which ~ simultaneously focuses all resolved masses onto one plane, allowing the integrating properties of an ion-sensitive emulsion to be used as the detector. Although electrical detection with an electron multiplier can be applied,6 the ion-sensitive emulsion-coated glass "photographic" plate is the most common method of detection and will be described in this article. Basic Principles General TechniqueDesmption

A schematic diagram of a spark source mass spectrometer is shown in Figure 1. The material to be analyzed forms the two electrodes separated by a spark gap. A pulsed 500-kHz high-voltage discharge across the gap volatilizes and ionizes the electrode material. The positive ions released are accelerated to 20-30 kV and passed into the mass spectrometer for energy and direction focusing. The electrostatic analyzer passes ions with an energy spread of about 600 eV, and focuses the beam onto a slit monitor that intercepts a constant fraction of the ions. This allows an accurate measurement of the number of ions (as Coulombic charge) entering the magnetic sector to be separated according to their mass-to-charge ratios and subsequently refocused and collected on the ion-sensitive plate detector. Figure 2 shows an example of SSMS data recorded on such a detector. The position of the collected ions (in the form of a line image of the source slit) provides qualitative identification of the isotopic masses (note the mass scale added to the plate to aid identification), and 600


Chapter 10

Figure 2

Ion-sensitive plate detector showing the species produced by the SSMS analysis of a Y203:Eu203mixture compacted with gold powder.

the blackness of the lines can be related to the number of ions striking a position (i.e., the concentration). Because the beam monitor allows accurate measurement of the total number of ions that are analyzed, a graded series of exposures (Le., with varying numbers of ions impinging on the plate) is collected, resulting in the detection of a wide range of concentrations, from matrix elements to trace levels of impurities. In Figure 2, the values of the individual exposures have been replaced with the concentration range that can be expected for a mono-isotopic species just visible on that exposure. In this example, exposures from a known Pt sample have been added to determine the response curve of the emulsion. Sample Requirementsand Examples

For bulk conductors and semiconductors, sample preparation consists of breaking, cutting, or sawing the solid approximately into the dimensions 1/ 16 in. x 1/ 16 in. x 1/ 2 in. Large, irregular sample electrodes can be accommodated; however, they often shield the path of ions into the mass spectrometer, thereby reducing the beam current and increasing the analysis time. If the preparation, handling, or packaging contaminates the sample electrodes with elements that are not of interest, they can be degreased in high purity methanol, etched in an appropriate semiconductorgrade acid, and washed in several portions of methanol before analysis. The final, and most critical cleaning is performed by presparking all electrode surfaces that will be consumed, before collecting the actual exposures for analysis. The presparking removes in vacuum the outer layers of the sample, which may contain trace levels of contamination due to handling or atmospheric exposure. This step also coats the surfaces in the analysis chamber to minimize memory effects, i.e., to minimize the chances of detecting elements from a previously analyzed sample. Cryosorption pumping’ using an AI203 or charcoal-coated plate filled with liquid nitrogen is also used during presparking and analysis to maintain reproducible source pressures and to reduce hydrocarbon interferences.

10.6

SSMS

601

+ IO TOWS FOR 2 MIN. RELEASE SLOWLY

TIPPED ELECTRODES

( + A g I F NON-CONDUCTIVE)

Figure 3

Schematic of a polyethyleneslug die used to compact powder samples. When the amount of materialavailable for analysis is small, tipped electrodescan be formed.

Table 2 is a typical example of SSMS analysis of a high-purity Pt wire that was simply broken in two and presparked. The elements from Be to U were determined in a single analysis requiring a total time of 1-2 hours. Hydrogen and Li must be measured in a separate set of exposures using a lower magnetic field and therefore generally are not included in a standard SSMS analysis. Because detection limits vary with plate sensitivity, background, isotopic abundance, and elemental mass, individual limits are listed for those elements not detected and are noted as less than (c). For practical reasons, a factor of about 3 better detection limits (3x longer andysis time) is generally the limit for this technique. Powders or nonconductors represent important forms of materials that are well suited for SSMS analysis. Powders of conductive material generallycan be prepared without a binder, but insdators first must be ground to a powder with a mortar and pestle, such as boron carbide and agate, then mixed with a high-purity conductive binder, such as Ag powder or graphite powder, and pressed to form solid, conductive electrodes. To prevent contamination from the metal die, a polyethylene cylinder is drilled to hold the powder such that the tips and sides of the electrodes touch only the polyethylene and not the steel parts ofthe die. If the sample material is limited in quantity, s m a l l portions (1-10 mg) can be tipped onto the end of high purity Ag.A die and tipped electrodes are shown diagrammaticallyin Figure 3. Of course, this procedure for nonconducting powders dilutes the sample, causing poorer detection limits and limiting the purity that can be specified to that of the binder. Although SSMS cannot be considered a surfice technique due to the 1-5 pm penetration of the spark in most materials, few other techniques can provide a trace elemental survey analysis of surfices consisting of films or having depths of interest 602


Chapter 10

Be B C'

5 0.003

w

0.1

OX

3 0.004

0.003

G

Rh

3

Pd Ag Cd

4 e 0.04

In Sn

e 0.04

e 0.02 < 0.04 < 0.02 < 0.02 e 0.02

F Na Mg

0.02

Sb Te I

0.4

Cs

Si

0.2 2

Ba La

P

0.04

ce

S

0.04 0.03 < 0.02 5 50.03

Pr Nd Sm Eu Gd

0.4

To DY

AJ

c1 K Ca sc Ti V Cr Mn Fe

0.02 50.3 0.07

4

Ho Er Tm Yb

0.1

0.2

< 0.02 < 0.02 e 0.02 e 0.07

e 0.06 e 0.04 e 0.07

< 0.02 e 0.07

e 0.02 < 0.06

e 0.03 e 0.07

Ga Ge

0.02 0.G 3 0.03 0.02 < 0.02

Re

e 0.04

As

0.02

os

.e 0.2

Se Br Rb Sr

< 0.02 < 0.02 < 0.01

Ir Pt

< 0.3 Major 13 e 0.2 e 0.08 .e 0.7

co

Ni cu Zn

Y Zr Nb Mo

50.02 e 0.008 3

Lu Hf

< 0.03 e 0.06

Ta** W

4 < 0.08

Au Hg TI

< 0.009

Pb Bi

50.2

Th

e 0.2 e 0.04

Ru U c 0.04 0.2 * ripper limits; source not baked to reduce background. SSMS annlysisof hlgh-purity Pt metal.

Table 2

10.6

SSMS

603

of -5 pm. Although the penetration depth can be somewhat controlled by the spark gap voltage, the electrode separation, and the speed of sample scanning, 1-5 pn is the typical range of penetration depths that can be achieved without punching through to deeper layers. Figure 3 shows a method for surface analysis using a highpurity metal probe (Au) as a counter electrode to spark an area of a sample's surface. The tip of the probe is positioned on the axis of the mass spectrometer, and the sample is scanned over the probe tip to erode tracks across the surface. By scanning over areas of about 1 cm2, detection limits on the order of 1 ppma can be achieved. By combining this survey surface analysis with depth profiling techniques, such as Secondary-Ion Mass Spectrometry (SIMS) or Auger Electron Spectroscopy (AES), elements of interest can be identified by SSMS and then profiled in detail by the other methods. The SSMS point-to-plane surhce technique has been shown to be particularly useful in the survey analysis of epitaxial films, heavy metal implant contamination, diffusion furnace contamination, and deposited metal layers. Data Evaluation

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution AUdMofabout 1500. For example, at mass M =60 a differenceof 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. The spark source is an energetic ionization process, producing a rich spectrum of multiply charged species (M/2, M / 3 , M / 4 , etc.). These masses, falling at halves, thirds, and fourths of the unit mass separation can aid in the positive identification of elements. In Figure 2, species like Au'~and Y+2are labeled. The most abundant species (matrix elements and major impurities) also form dimers and trimers (and so forth) at two and three times (and so forth) the mass of the monomer. Although these species can cause interference with certain trace elements, they also can aid in positively identifylng a particular element. Finally, the spectrum generally contains mixed polyatomic species, such as MO', M o a + , MC' (in graphite), and MAg' (in silver).All such possibilities must be considered in the qualitative interpretation of a spark source mass spectrum. Of course, the most reliable and accurate method of quantitative analysis is to calibrate each element with standards prepared in matrices similar to the unknown being analyzed. For a survey technique that is used to examine such a wide variety of materials, however, standards are not available in many cases. When the technique is used mainly in one application (typing steels, specifying the purity of alloys for a selected group of elements, or identlfying impurities in silicon boules and 604


Chapter 10

wafers), such standards can be developed and should be applied. Because of the erratic nature of the spark (in terms of time) and variability in the response of the ion-sensitive emulsion detector, accuracy using standards to generate relative sensitivity factors is generally within 20-50%. Due to the relative uniformity of ion formation by the RF spark (although its timing is erratic), the most widely used method of quantitation in SSMS is to assume equal sensitivity for all elements and to compare the signal for an individual element with that of the total number of ions recorded on the beam monitor. By empirically calibrating the number of ions necessary to produce a certain blackness on the plate detector, one can estimate the concentration. The signal detected must be corrected for isotopic abundance and the known mass response of the ion-sensitive plate. By this procedure to accuracies within a factor of 3 of the true value can be obtained without standards. The optical density (blackness) of the lines recorded can be measured most accurately using a microdensitometer to scan each line and measure the transmission of light through it. A set of known relative exposures (from charge acccmulated by the mass spectrometer beam monitor and known isotopic ratios) is used to establish the emulsion response curve relating transmission to exposure. The absolute position of this response curve on the exposure axis can be determined using standards or from isotopes of a pure element. For concentration determinations requiring the highest precision, the microdensitometer approach is recommended. This method, however, is time consuming; it can be considerably shortened by a well-established “visualy7 method. If a graded series of exposures is made in relative steps of 1,2,5, 10,20, 50, etc. (see the graded series of exposures in Figure 2), the exposure necessary to produce a barely detectable line for a particular isotope can be determined by simply obsenring a well-lighted plate with a 3-5x eyepiece. By determining the average exposures for which barely detectable lines appear for known concentrations of some elements in various matrices, a particular instrument can be calibrated to provide estimates of concentration without further analysis of standards, except to occasionally check the relationship between the beam monitor and the emulsion response. This visual method is surprisingly consistent when care is taken to provide accurate relative exposures, and it produces values that are generally accurate within a hcror of 3. Several elements, such as Na, K,Cayand Al, are best estimated using a multiply charged species (+2 in most cases). For the alkaline and alkaline earth elements in particular, the number of singly charged ions can be greatly enhanced by thermal excitation; a more accurate assessment is made by measuring the +2 species and applying an empirically determined correction factor. The accuracy for elemental concentrations determined in this manner is generally within a factor of 10.

10.6

SSMS

605

CharaCtuiStiC

Es

Detection limits

1-10 ppm

0.00001-0.01 pprn

0.003-0.03 pprn

Concentration

Minor, trace

Major, minor, Ultra-trace

Minor, trace

Elemental coverage

Metals

All elements

All elements

Accuracy without standards with standards

f1 0 ~ f 20%

f 3x f lo-20%

f 3x f 20-50%

Matrix effects

Strong

Weak

Weak/ medium

Bulk/ surface

Bulk

Bulk

Bulk and surfice

Conductivity

Conductor or insulator

Conductor: runasis Insulator: +Ag

Conductor: run as is Insulator: +AgorC

Sample Form

Solid/ powder

1 Conductingpin (shape important)

2 Conductingpins (can be irregular)

Table 3

SSMS

GDMS

SSMkomparison with other techniques.

Comparison With Other Techniques

Although numerous analytical techniques have been developed for the quantitative determination of specific elements at trace levels in solids, the three most-used techniques providing multi-element surveys are Emission Spectroscopy (ES), Glow Discharge Mass Spectrometry (GDMS), and SSMS. GDMS is covered in detail elsewhere in this volume, but it is instructive to compare these techniques in tabular form. Table 3 provides this comparison for a number of characteristics that should be considered when choosing a technique. In most situations the required detection limits clearly define one’s choice. Elemental coverage is important when nonmetals, such as As, P, C1, F, C, and 0,play a role as trace elements, making the mass spectrometric techniques a clear choice. Sample shape and form are also issues that must be considered. The versatility of SSMS in accommodating a wide variety of materials while maintaining high sensitivity for all elements is one of its prime features. Inductively Coupled Plasma-Optical (ICP-optical) methods and ICPMS are extremely sensitive elemental survey techniques that also are described in this volume. ICP methods, however, require a solution for analysis, so that the direct 606


Chapter 10

COUNTER ELECTR0DE

/

SCAN SAMPLE

4

\

GAP CONTROL MOVE

SAMPLE

Figure 4

SSMS surface analysis. The point-to-planetechnique allows ppma elemental surveys over a depth of 1 6 pm.

examinationof solids is not possible. Because solution techniques offer relative ease of preparing standards, ICP-optical methods and ICPMS might be chosen in cases where accuracy is most important and the solids can be dissolved without contamination. Conclusions

SSMS can provide a complete elemental survey with detection limits in the 1050ppba range and can deal with a wide variety of sample types and forms. Although GDMS offers higher sensitivity and accuracy of 20%, SSMS is still the technique of choice in many situations. Materials, such as carbon, that do not sputter rapidly enough for good GSMS detection limits, and insulators that cause erratic sputtering when combined with a conductive powder, are excellent candidates for SSMS analysis. In addition, the point-to-plane surface method is one of the few techniques available that can provide a complete elemental survey of 15 pn thick films with detection limits on the order of 1 ppma (see Figure 4). Having described SSMS in some detail as a very useful technique for trace elemental survey analysis, one must note that the lack of manufacture of new instruments and the rising development of GDMS limit its future use. Industrial and service laboratories having SSMS instruments and experienced personnel will continue to use SSMS very effectively. Where there is the need for increased sensitivity, reaching detection limits of less than 1 ppba, and where there is sufficient justification to warrant the cost of GDMS ($600,000-8700,000 for magnetic sector instruments, and about $250,000 for quadrupole instruments), it is anticipated that SSMS gradually will be replaced. With progress being made in the instrumentation and methodology of GDMS, there are currently very few instances where 10.6

SSMS

607

GDMS cannot be used instead of SSMS. As GDMS source designs are developed to allow dean, thin-film analyses, and some limitations are accepted for the analysis of insulators, GDMS instrumentation will replace more and more of the older SSMS installations. For the present, however, there are excellent laboratories having SSMS instrumentation and services, and SSMS should be used when it proves to be the technique of choice. Related Articles in the Encyclopedia

GDMS, ICPMS, and ICP-Optical References 1

2

3

4

5

J. Franzen and H. Hintenberger. Zeit.firNatu@rschung. 189,397, 1963. J. R Woolston and R E. Honig. Rev. Sci. Imtr. 35,69, 1964. J. R Woolston and R. E. Honig. In Proceedings of the Elmenth Annual Confiwnce on Mass Spectrometry undAl1ied Topics. San Francisco, 1963. J. R Woolston and R. E. Honig. In Proceedings of the EleventhAnnual Confirerace on Mars Spectrometry a d d l i e d Topics. Montreal, 1964. J. Mattauch and R F. K. Herzog. Z Pbysik. 89,786, 1934.This is the original paper showing the double-focusing geometry necessary to focus onto a plane rather than at a single point.

6

C. W. Magee. CriticalPammeters Afecting Precision andAccuracy in Spark Source Mars Specnometry with EaCtrical Detection. PhD thesis, University of Virginia, University Microfilms, Ann Arbor, MI, 1973.

7

W. L. Harrington, R K. Skogerboe, and G. H. Morrison. Anal. C h m . 37, 1480-1484,1965. The use of cryosorption pumping for SSMS is demonstrated.

608


Chapter 10

10.7

GDMS Glow-Discharge Mass Spectrometry J O H N C. HUNEKE A N D WOJCIECH VIETH

Contents

Introduction Basic Principles Sample Preparation and Analytical Protocol GDMS Quantitation Applications Conclusions

Introduction

Glow-Discharge Mass Spectrometry (GDMS) is a mass spectrometric analytical technique primarily used to measure trace level impurities in conducting or semiconductingsolids. It can also be used, but less commonly, for elemental depth profde analyses. The primary advantages of GDMS are its sensitivity (ppt detection limits in some cases); its quantitative accuracy (20% on average), achieved without complicated standardization procedures; and its ability to detect essentially all elements in the periodic table from lithium to uranium at approximately the same sensitivity. Because GDMS can provide ultratrace analysis with total elemental coverage, the technique fills a unique analytical niche, supplanting Spark-Source Mass Spectrometry (SSMS) by supplying the same analysiswith an order-of-magnitudebetter accuracy and orders-of-magnitude improvement in detection limits. GDMS analy-

609

sis has matured rapidly and has become more widely available since the recent introduction of commercial GDMS instrumentation.

Basic Principles

Ion Sources In general, all GDMS ion sources use a noble gas glow-discharge plasma sustained at about 1 Torr pressure and a few Watts of discharge power. A conducting solid sample for GDMS analysis forms the cathode for a DC glow discharge (Figure 1). Atoms are sputtered nonselectively fiom the sample surface by ions accelerated from the plasma onto the surfice by the cathode voltage. The sputtered atoms diffuse into the plasma and are mostly ionized by collision with metastable discharge gas atoms (so-called Penning ionization) but also in small part by electron impact. Penning ionization, more so than electron impact ionization, provides similar ionproduction efficiencies for the majority of the elements. Plasma ions extracted through the exit aperture, including the analyte ions, are electrostatically accelerated into a mass analyzer for measurement. Ion sources for GDMS have undergone significant evolution, ultimately resulting in dischargecells exposing only the sample (cathode)surfice and the metal interior of the Ta discharge cell (anode) to the plasma, a design that minimizes contamination and enhances reliability. The glow-discharge cells accept a variety of shapes and surfice conditions, requiring only sufficient length or diameter of the sample. Typically, pin- or wakr-shaped samples are required. It is very helphl to include cryocooling into the design of the discharge cell to enable the analysis of materials having low melting point and to reduce the density of molecular ions created from Yatmospheric”gas related contaminants in the glow-discharge plasma. At the current time, analytical glow-discharge sources incorporate a DC glow discharge. E f h t s have been underway to develop glow-dischargesources appropriate for the analysis of electrically insulating materials (e.g., glass and ceramic), which comprise a very important class of materials for which few methods are currently available fbr complete, I11-coverage analysis to trace levels. Two alternatives have been suggested as appropriate: RF-powered glow-discharge plasmas; and electron beam-assisted plasmas. While efforts are being made in both directions, no analytically viable sources have yet been made available. The sputter sampling of the exposed surfice also provides concentration depth profding capability for GDMS. Depth resolution of some 0.1 pm has been demonstrated, with 1-2 orders of magnitude dynamic range due to geometric limitations and the high operating pressure of the glow-discharge source. With a rapid sputtering rate of about 1 p / m i n , GDMS is particularly usem for thick-film (10100 pm) depth profiling. GDMS can provide accurate, sensitive, and matrix inde-

610


Chapter 10

b

Figure 1

Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

pendent concentration depth profiles throughout a complex film structure, including interfaces. Mass Spectrometers

Demonstration of GDMS feasibility and research into glow-dischargeprocesses has been carried out almost exclusively using the combination of a glow-discharge ion source with a quadrupole mass spectrometer (GDQMS). The combination is inexpensive, readily available and suitable for such purposes. In addition, the quadru10.7

GDMS

611

pole provides the advantage of rapid mass spectrum scanning for data acquisition. Because ion transmission is limited and significant molecular ion mass interferences are unresolvable with the low mass resolution capabilities of the quadrupole, elemental detection limits by GDQMS are only slightly better than those provided by Optical Emission Spectroscopy (OES) methods, and the analytical usellness of the GDQMS overlaps that of OES techniques (cf. Jakubowski, et al. '). Even so,the full elemental response of GDMS provides a substantial enhancement in analytical capability compared to the more selective OES. The introduction of GDMS instrumentation using high mass resolution, high-transmission magnetic-sector m a s spectrometers has circumvented the major limitations of the quadrupole, providing an instrument with sub-ppb detection limits, albeit at the expense of analytical time. GDMS instrument and source descriptions have recently been the subject of an extensive review by Harrison and Bentz.* The optimal analytical GDMS instrument for bulk trace element analysis is the one providing the largest analytical signal with the lowest background signal, the fewest problems with isobaric interferences in the mass spectrum (e.g., the interference of 40Ar160+ with 56Fe+),and the least contamination from instrument components or back contamination from preceding sample analyses. The first commercial GDMS instrument incorporated a high mass resolution magnetic-sector mass spectrometer to enable interfering isobaric masses to be eliminated, while at the same time providing high u s e l l ion yields. The ion detection system of this instrument combined a Faraday cup collector, for the direct current measurement of the large ion beam associated with the matrix element, with a single-ion counting capability to measure the occasional trace element ion. The resulting ion current measuring system provides the necessary large dynamic range for matrix to ultratrace level measurements. Instrument configurations other than a magnetic-sector mass spectrometerwith a pin sample source are also suitable fbr analytical GDMS, but with some compromise in analytical performance. If analysis to ultratrace levels is not required, but only measurements to levels well above the background of isobaric mass spectral interferences, low-resolution quadrupole mass spectrometer based instruments can be configured. Such instruments have recently been made available by several instrument manufacturers. In these cases, the unique advantage of GDMS 1'ies not with the ultratrace capability but with the full elemental coverage from matrix concentrations to levels of 0.0 1-0.1 ppm. Also, quadrupole MS mass spectral analysis requires significantly less time, enabling the more rapid analysis suitable for depth profiling of films. Sample Preparation and Analytical Protocol

Accurate GDMS analysis has required the development of analytical procedures appropriate to the accuracy and detection limits required and specific to the mate612


Chapter 10

rial under analysis. Protocol particulars will differ from laboratory to laboratory. To use GDMS to advantage (Le., to improve measurements to the ppb level) the surface exposed to the sampling plasma must be very clean. Common methods for surface cleaning are chemical etching and electropolishing using high-purity solutions. If such cleaning is not feasible (e.g., for pressed powders), presputtering of the surface in the glow-discharge source or with a separate sputtering unit to remove contaminants prior to analysis is generally required. (This procedure could not, of course, be used prior to concentration depth profiling measurements.) The risk of recontamination to ppb levels is high, and care must be taken in rinsing, handling, and transporting the cleaned sample. The composition of the sample measured by GDMS reflects the surfice composition, and the argument must be made that the measurement is representative of the bulk. This requires thorough sputter cleaning of residual contaminants and sputter equilibrium of the phases exposed at the surface. The pragmatic (and reasonably conservative) criterion that both goals have been accomplished is that the same composition has been obtained in consecutivemeasurements during the analysis (ie., a “confirmed” analysis). Other than these requirements, the analysis protocol must be suitable for the instrument and the detection limits required, since in many instances the detection limit arises from lack of signal and not from backgrounds or interferences. Accurate final results from GDMS are available very quickly. Samples for GDMS analysis requires little preparation other than shaping and cleaning, although the cutting of a pin or wafer from extremely hard materials can be time consuming. The actual GDMS analysis takes only on the order of one to two hours, depending on elemental coverage and detection limits required. Data reduction is on-line and essentially immediate. GDMS Quantitation

Pragmatically, the relative concentrations of elements are determined from the measured ion beam ratios by the application of relative sensitivity factors, which are determined experimentally from standard samples:

where Mand Nare the elements of concern, l i s the measured ion current (including all isotopes of the element), and RSFNCM) is the relative sensitivityfactor for A4 relative to N. Vieth and Huneke3 have recently presented a thorough discussion of GDMS quantitation, including the measurement of relative GDMS sensitivity factors and a modeling of glow-discharge source processes to enable semiempirical estimates of 10.7

GDMS

613

CHEMICAL QROUP

Figure 2

Elementalrelative sensitivity factors as a function of chemical groupingin the periodic table. The factors are determined from the measurement of 30 standard samples representing 6 different matrix elements. The factors are matrix-independent and similar within a factor of 10, except for C and 0. There is a pronounced trend across the groupd elements, with a similar but separate trend across the group-A elements.

RSFs. Figure 2 exhibits the average RSFs determined from analyses of 30 different standard samples of seven different matrices. The RSFs are matrix independent, with the spread in RSF determinationsbeing due primarily to standard alloy inhomogeneities. It is remarkable that for all but N and 0 the factors range over only a decade. 614


Chapter 10

The sensitivityfactors summarized in Figure 2 are appropriate for analyses using a particular instrument (the VG 9000 GDMS) under specific glow-discharge conditions (3 mA and 1000 V in Ar,with cryocooling of the ion source) and with a well-controlled sample configuration in the source. RSFs depend on the samplesource configuration. In particular, they vary significantlywith the spacing between the sample and the ion exit aperture from the cell. Use of the factors shown in Figure 2 under closely similar conditions will result in measurements with 20% accuracy. These factors can also be used to reduce the data obtained on other instruments, but the accuracy of the results will be reduced. In particular, the factors shown can be only approximately d i d for results obtained using a quadrupole mass spectrometer, since ion-transmission characteristics differ significantly between the quadrupole mass spectrometer and the magnetic-sector spectrometer used to obtain the results of Figure 2. It is clear from the RSF data shown in Figure 2 that even without the use of RSFs, a semiquantitativeanalysis accurate to within an order of magnitude is quite possible, and GDMS indeed will provide 111 coverage of the periodic table. The analysis of a material of unknown composition will be elementally complete to trace levels, with no glaring omissions that may eventually return to haunt the end user of the material. Applications

The application of GDMS is strongest in the areas of: 1 Qualification of 5N-7N pure metals, since GDMS provides f dlelemental coverage to ultratrace levels z The analysis of specific elements for which GDMS is particularly well suited compared to other methods (e.g., measurement to ppb levels of S, Se, Te, Pb, Bi, TI, in high-performance alloys; and measurement of U and Th in sputter targets) 3

The analysis of a material known to be impure, but with unspecified impurity elements

4

The analysis of a material in limited supply, and when too little is available for analysis by alternative methods.

A number of examples of the application of GDMS to various metals and alloys are exhibited below. All measurements were performed using the VG 9000 GDMS instrument with standard glow-discharge conditions of 3-mA discharge current and 1000-V discharge voltage except as noted. The standard pin dimensionswere a diameter of 1.5-2.0 mm and a length of 18-22 mm. Indium and Gallium Metal

Table 1 summarizes the results of an analysis to the 6N-7N total impurity level of very high purity In and Ga metals, such as would be used in the manufacture of III10.7

GDMS

615

Concentration (ppmw) Element

Li Be B C N 0 F Na

Mg

AI Si P S c1 K Ca

In

8 ev) or high excitation energies emit most intensely in the normal analyticalzone. Emission usually is collected from a 3-5 mm section of the plasma near the peak emission intensity. Emission from atoms with low ionization energies and low excitation energies (Li, NayK, Cs, and Rb) is most intense lower in the plasma. Unlike ion emission intensities, the atomic emission intensity peak location is a strong function of ionization and excitation energies. Usually, the ultraviolet and visible regions of the spectrum are recorded. Many of the most intense emission lines lie between 200 nm and 400 nm. Some elements (the halogens, B, C, P, S, Se, As, Sn, N, and 0)emit strong lines in the vacuum ultraviolet region (170-200 rim), requiring vacuum or purged spectrometers for optimum detection. Quantitation

Calibration curves must be made using a series of standards to relate emission intensities to the concentration of each element of interest. Because ICP-OES is relatively insensitive to matrix effects, pure solutions containing the element of interest often are used for calibration. For thin films the amount of sample ablated by spark discharges or laser sources is often a strong function of the sample’s composition. Therebre, either standardswith a composition similar to the sample’s must be used or an internal standard (a known concentration of one element) is needed. 636


Chapter 10

0.1-1

1-10

10-50

50-100

10CL500

PPb

PPb

PPb

PPb

PPb

Ba

A%

Li

Al

Nb

As

Rb

Be

B

Lu

AU

Nd

K

U

Ca

co

Mo

Bi

Ni

P

Mn

cu

Re

C

Pb

Pd

sc

Pr

S

Pt

Se

Cr

Ti

ce cs

Eu

V

DY

Rh

Te

Fe

Y

Er

Ru

T1

Ho

Yb

Ga

Sb

I

Zn

Ge

Si

La

Zr

Gd

Sm

Hf

Sn

Hg

Ta

In

Tb

Ir

Th

Na

W

Mg Sr

Table 1

Typical detsction limits (ppb) for ICP-OES(using a pneumatic nebulizer for sample introduction)of the most sensitive emission line between 175 nm and 850 nm for each element.

Detedon Limits

Typical elemental detection limits are listed in Table 1. The detection limit is the concentration that produces the smallest signal that can be dknguished from background emission fluctuations. The continuum background is produced via radiative recombination of electrons and ions (M++ e-+ M +bv or M ++ e-+ e-+ M + e-+ hv). The structured background is produced by partially or completely overlapping atomic, ionic, or in some cases, molecular emission. To obtain precision better than 10%the concentration of an element must be at least 5 times the derection limit. 10.9

ICP-OES

637

Detection limits for a particular sample depend on a number of parameters," including observation height in the plasma, applied power, gas flow rates, spectrometer res~lution,~ integration time, the sample introduction system, and sample-induced background or spectral overlap^.^ Sample Introduction

Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid partides 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. Introduction of Liquids and Solutions ofDissolved Solids

Most often samples are introduced into the plasma as a liquid aerosol. Solid samples are dissolved using an appropriate procedure. Pneumatic nebulizers of various designs'y2 generate aerosols by pumping or aspirating a flow of solution into a region of highly turbulent, high-speed gas flow. Concentric cross-flow nebulizers are used for solutions having less than 1% dissolved solids. V-groove (Babington) nebulizers can be used for highly viscous solutions having a high dissolved solid content. Most of the droplets produced by these nebulizers are too large to be vaporized effectively in the plasma; therefore, a spray chamber is used to remove large droplets via gravity and to cause them to impact onto the walls. The smallest droplets are able to follow the gas flow into the plasma. Typically only 1-2% of the sample reaches the plasma, and liquid sample volumes of 1 mL or more are required. Flow injection techniques can be used to inject sample volumes as small as 10 pT., into a flowing stream of water with little degradation of detection limits. Frit nebulizers', have efficiencies as high as 94% and can be operated with as little as 2 pL of sample solution. Electrothermal vaporization's can be used for 5-1 00 pL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-hnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the tenter of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%.

638


Chapter 10

Direct introduction of Samples from Solids, Surfaces, or Thin Films

There are advantages to direct solid sampling. Sample preparation is less time consuming and less prone to contamination, and the analysis of microsamples is more straightforward. However, calibration may be more difficult than with solution samples, requiring standards that are matched more closely to the sample. Precision is typically 5% to 10% because of sample inhomogeneityand variations in the sample vaporization step. In the direct insertion technique,'?2, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selectivevolatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and highly automated systems have been des~ribed.~ Arc and spark discharges have been used to ablate material from a solid conducting sample surface. The dry aerosol is then transported to the plasma through a tube. Detection limits are typically in the low ppm range. The precision attainable with spark discharges that sample over a relatively large surface area (0.2-1 cm2) is typically 0.5% to 5.0%. Calibration curves are linear over at least 3 orders of magnitude, and an accuracy of 5% or better is realized. Commercial instruments are available. In some cases it is possible to use pure aqueous standards to produce the calibration curves used for spark ablation ICP-OES. In general, calibration curves for spark or arc ablation followed by ICP-OES are more linear and less sample matrix-dependent than calibration curves in spark or arc emission spectrometry. A vapor sample and dry aerosol also can be produced from surfaces via laser ablation.', Typically, solid state pulsed Nd-YAG, Nd-glass, or ruby lasers have been used. The amount of material removed from the sample surface is a function of the sample matrix and the laser pulse energy, wavelength and focusing, but is usually in the pm range. Part-per-million detection limits are possible, and the technique is amenable to conducting and nonconducting samples. Precision is typically 3% to 15%. Shot-to-shot laser pulse energy reproducibility and sample heterogeneity are the two main sources of imprecision in this technique.

',*

Instrumentation-Detection Systems

Three different types of grating spectrometer detection systems are used (Figure 3): sequential (slew-scan) monochromators, simultaneous direct-reading polychroma10.9

ICP-OES

639

I E P L

C

Figure 3

Grating spectrometers commonly used for ICP-OES: (a) monochromator, in which wavelength is scanned by rotatingthe gratingwhile using a single photomuttiplier tube (PMT)detector; (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40or more exit dits and PMTs can be arranged along the focal plane); and (e) spectrally segmented diode-array spectrometer.

tors, and segmented diode array-based spectrometers. The choice detection system depends on the number ofsamples to be analyzed per day, the number of elements of interest, whether the analysis will be ofsimilar samples or of a wide range of sample types, and whether the chosen sample-introductionsystem will produce steadystate or transient signals. Slew-scan spectrometers (Figure 3a) detect a single wavelength at a time with a single photomultiplier tube detector. The grating angle is rapidly slewed to observe a wavelength near a n emission line from the element of interest. A spectrum is acquired in a series of 0.01-0.001 nm steps. The peak intensity is determined by a fitting routine. Background emission can be measured near the emission line of interest and subtracted from the peak intensity. The advantage of slew-scan spectrometersis that any emission line can be viewed, so that the best line for a particular sample can be chosen. Their main disadvantage is the sequential nature of the multielement analysis and the time required to slew fiom one wavelength to another (WicaUy a few seconds). 640


Chapter 10

Direct-reading polychromators'. (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrumenr is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and s o h a r e are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. The spectrally segmented diode-array spectrometer5 uses three gratings to produce a series of high-resolution spectra, each over a short range of wavelengths, at the focal plane (Figure 34. A 1024-element diode array is used to detect the spectra simultaneously. By placing the appropriate interchangeable mask in the focal plane following the first grating, the short wavelength ranges to be viewed are selected. The light is recombined by a second grating, forming a quasi-white beam oflight. A third grating is used to produce high-resolution spectra on the diode array. It is much easier to change masks in this spectrometer than to reposition exit slits in a direct-reading polychromator. The diode array-based system also provides simultaneous detection of the emission peak and nearby background. This capability is particularly advantageous when using a sample-introduction technique that generates a transient signal. Limitationsand Potential Analysis Errors

One of the major problems in ICP-OES can be spectral overlaps.'. 2, Some elements, particularly rare earth elements, emit light at thousands of different wavelengths between 180 nm and 600 nm. Spectral interferences can be minimized, but not eliminated, by using spectrometers with a resolving power (A/ AL) of 150,000 or higher.'. If a spectral overlap occurs, the operator can choose a different line for analysis; or identifj.the source of the interfering line, determine its magnitude, and subtract it from the measuring intensity. Tables of potential spectral line overlaps for many different emission lines are available.6i Some manufacturers provide computer database emission line lists. Most commercial direct-reading polychromators include software to subtract signals due to overlapping lines.' This is effective if the interferant line intensity is not large compared to the elemental line of interest and another line for the interferant element can be measured. Although nonspecrral interference effects are generally less severe in ICP-OES than in GFAA, FAA, or ICPMS, they can occur." 23 In most cases the effects produce less than a 20% error when the sample is introduced as a liquid aerosol. High concentrations (500 ppm or greater) of elements that are highly ionized in the

'

10.9

ICP-OES

641

72

72

f 8 os

8 os

0

0 6

10

76

20

26

MnMt &ow &ti cor7 bmd

Figure 4

SO

5

70 Jiei#t

75

20

above &ti

o0.9 f

26

50

i

Effect of matrix on Sr ion emission at different heights in the plasma. Samples contained 50 ppm Sr in distilled, deionized water: (a) emission in the presence and absence of NaCl (solid line-no NaCl added; dashed line-O.05 M NaCl added); and (b) effect of the presence and absence of HCI (solid l i n p n o HCI added; dashed l i n H . 6 M HCI added).

plasma can affect emission intensities. The magnitude and direction of the effect depends on experimental parameters including the observation height in the plasma, gas flow rates, power, and, to a lesser degree, the spectral line used for analysis and the identity of the matrix. A location generally can be found (called the cros-over point) where the effect is minimal (Figure 4a). If emission is collected from a region near the cross-over point, errors due.to the presence of concomitant species will be small (generallyless than 10% or 20%). The presence of organic solvents (1% by volume or greater) or large differences in the concentration of acids used to dissolve solid samples can also affect the emission intensities (Figure 4b).'. 2, Direct solid-sampling techniques generally are more susceptible to nonspectral interference e a c t s than techniques using solutions. The accuracy can be improved through internal standardization or by using standards that are as chemically and physically similar to the sample as possible. Errors due to nonspectral interferences can be reduced via matrix matching, the method of standard additions (and its multivariant extensions), and the use of internal standards.'. 2, Applications

ICP-OES has been applied to a wide range of sample types, with no single area or technology dominating. Elemental analysis can be performed on virtually any sample that can be introduced into the plasma as a liquid or dry aerosol. Metals and a wide variety of industrial materials are routinely analyzed. Environmental samples, including water, waste streams, airborne particles, and coal fly ash, are also amenable to ICP-OES. Biological and clinical samples, organic solvents, and acids used in semiconductor processing are widely analyzed. 642


Chapter 10

Laser-ablation ICP-OES has been used to analyze metals, ceramics, and geological samples. This technique is amenable to a wide variety of samples, including surfaces and thin films (pm depths analyzed), similar to those analyzed by laser microprobe emission techniques (LIMS). However, interference effects are less severe using separate sampling and excitation steps, as in laser-ablation ICP-OES. Laser-ablation ICPMS is becoming more widely used than laser-ablation ICP-OES because the former's detection limits are up to 2 orders of magnitude. Spark discharge-ablation ICP-OES is used mainly to analyze conducting samples. Conclusions

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet-visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. Related Articles in the Encyclopedia

ICPMS, GDMS, SSMS, and LIMS References 1

l? W. J. M. Boumans. Inductive4 Coupled Plasma Emission Spectroscopy, Parts i a n d II. John Wiley and Sons, New York, 1987. An excellent description of the fundamental concepts, instrumentation, use, and applications of ICP-OES.

2

3

4

A. Montaser and D. W. Golightly. Inductively Coupled Plasma in AnalyticalAtomic Spectromeq. VCH Publishers, New York, 1987. Covers similar topics to Reference 1 but in a complementary manner. l? W. J. M. Boumans and J. J. A. M. Vrakking. Spect. Acta. 42B, 819, 1987. Describes how spectrometer resolution affects detection limits in the presence and absence of spectral overlaps. W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of IO-@ liquid samples or small amounts (10 mg) of powder samples.

10.9

ICP-OES

643

5

6

7

8

9

G. M. Levy,A. Quaglia, R E. Lazure, and S. W. McGeorge. Spect. ACM. 42B,341, 1987. Describes the diode array-based spectrallysegmented spectrometer for simultaneousmultielement analysis. I? W. J. M. Boumans. Line Coincidence Tablesfir Inductively Coupkd P h m a Atomic Emission Spectrometry.Pergamon Press, Oxford, 1980, 1984. Lists of emission lines fbr analysis and potentially overlapping lines with relative intensities, using spectrometers with two different resolutions. R K. Winge, V A. Fassel, V J. Peterson, and M. A. Floyd. Inductively Coupled Plasm Atomic Emission SpectroscopyAn Atlas of Spectral Infirmation. Elsevier, Amsterdam, 1985. ICP-OES spectral scans near emission lines useful for analysis. R I. Botto. In: Developments in Atomic P h m a SpectrocbemicalAdysis. (R. M. Barnes, ed.) Heyden, Philadelphia, 1981. Describes method for correction of overlapping spectral lines when using a polychromator for ICP-OES. J. W Olesik. h l y t . G e m . 63,12A, 199 1. Evaluation of remaining limitations and potential sources of error in ICP-OES and ICPMS.

644


Chapter 10

NEUTRON AND NUCLEAR TECHNIQUES 11.1 11.2 11.3 11.4

11.O

Neutron Diffraction 648 Neutron Reflectivity 660 Neutron Activation Analysis, NAA 671 Nuclear Reaction Analysis, NRA 680

INTRODUCTION

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton accelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth, Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since N M is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. Neutron diffraction uses neutrons of wavelengths 1-2 A, similar to those used for X-rays in XRD (Chapter 4), to determine atomic structure in crystalline phases in an essentially similar manner. There are several differences that make the techniques somewhat complementary, though the need to go to a neutron source is a significant drawback. Because neutrons are diffracted by the nucleus, whereas X-ray diffraction is an electron density effect, the neutron probing depth is about lo4 longer than X-ray. Thus neutron diffraction is an entirely bulk method, which can be used under ambient pressures, and to analyze the interiors of very large samples, or contained samples by passing the neutron flu through the containment walls. Along with this capability, however, goes the difficulty of neutron shielding and safety. Where X-ray scattering cross sections increase with the electron density of the atom, neutron scattering varies erratically across the periodic table znd is 645

approximately equal for many atoms. As a result, neutron diffraction “sees” light elements, such as oxygen atoms in oxide superconductors, much more effectively than X-ray diffraction. A further difference is that the neutron magnetic moment strongly interacts with the magnetic moment of the sample atoms, allowing determination of the spatial arrangements of magnetic moments in magnetic material. The equivalent interaction with X rays is a factor of lo6 weaker. Neutron diffraction has proved useful in studying thin magnetic multilayers because, though it is a bulk technique, the magnetic scattering interactions are strong enough to enable usable data to be taken for as little as 500-Athicknesses for metals. In Neutron Reflectivity the neutron beam strikes the sample at grazing incidence. Below the critical angle (around 0.lo), total reflection occurs. Above it, reflection in the specular direction decreases rapidly with increasing angle in a manner depending on the neutron scattering cross sections of the elements present and their concentrations. On reaching a lower interface the transmitted part of the beam will undergo a similar process. H and D have one of the largest “mass contrasts” in neutron-scattering cross section. Thus, if there is an interface between a H-containing and a D-containing hydrocarbon, the reflection-versus-angle curve will depend strongly on the interface sharpness. Thus interdihsion across hydrocarbon material interfaces can be studied by D labeling. For polymer interfaces the depth resolution obtained this way can be as good as 10A at buried interface depths of 100 nm, whereas the alternative techniques available for distinguishing D from H at interfaces, SIMS (Chapter 10) and EM (Chapter 9), have much worse resolution. Also, neutron reflection is performed under ambient pressures, whereas SIMS and ERS require vacuum conditions. Labeling is not necessary if there is sufficient neutron “mass contract” already available-e.g., interhces between fluorinated hydrocarbons and hydrocarbons. The technique has also been used for biological films and, magnetic thin films, using polarized neutron beam sources, where the magnetic gradient at an interface can be determined. Though a powerful technique, Neutron Reflectivityhas a number of drawbacks. Two are experimental: the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation: the reflectivity-versus-angle data does not directly give a a depth profile; this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses, Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffusion. In NAA the sample is made radioactive by subjecting it to a high dose (days) of thermal neutrons in a reactor. The process is effective for about two-thirds of the elements in the periodic table. The sample is then removed in a lead-shielded container. The radioisotopes formed decay by B emission, y-ray emission, or X-ray emission. The y-ray or X-ray energies are measured by EDS (see Chapter 3) in spe646

NEUTRON AND NUCLEAR TECHNIQUES

Chapter l l

cial laboratories equipped to handle radioactive materials. The energies identrfy the elements present. Concentrations are determined from peak intensities, plus knowledge of neutron capture probabilities, irradiation dose, time from dose, and decay rates. The technique is entirely bulk and is most suitable for the simultaneous detection of trace amounts of heavy elements in non-y-ray emitting hosts. Since decay lifetimes can be very variable it is sometimes possible to greatly improve detection limits by waiting for a host signal to decay before measuring that of the trace element. This is true for Au in Si where levels of 3 x 1O7 atomskc are achieved. An As- or Sb-doped Si, host would give much poorer limits for Au, however, because of interfering signals from the dopants. In NRA a beam of charged particles (e.g., H, N, or F) from an ion accelerator at energies between a few hundred keV and several MeV (cf., RBS, Chapter 9) induces nuclear reactions for specific light elements (up to Ca). Various particles (protons, 01 particles, etc.) plus y-rays are released by the process. The particles are detected as in RBS and, similarly their yield-versus-energy distribution identifies the element and its depth distribution. This can provide a rapid nondestructive, analysis for these elements, including H. The depth probed can be up to several w with a resolution varying from a few tens of nanometers at the surface to hundreds of nanometers at greater depths. Usually there is no lateral resolution, but a microbeam systems with a few-micron capability exist. If particle detection is too ineficient (too low energies), y-ray spectroscopy (cf., N U ) can yield elemental concentration, but not depth distributions. For some elements the nuclear reaction process has a maximum in its cross section at a specific beam energy, ER (resonance energy). This provides an alternative method of depth profiling (resonance profiling), since if the incident beam energy, 4,is above ER, it will drop to ER at a specific reaction distance below the surface (electronic energy losses, see RBS). By changing 4 the depth at which ER is achieved is changed, and so the depth at which the analyzed particles are produced is changed. Resonanceprofiling can have better sensitiviry than nonresonance, but the depth resolution depends on the energy width of the resonance.

647

11.1

Neutron Diffraction R A Y M O N D G . TELLER

Contents

Introduction Basic Principles Neutron Sources Utility Conclusions

Introduction

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystallinematerial, the development of neutron diffraction as a usefd analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailabilityof neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. As with other d i h c t i o n techniques (X-ray and electron), neutron diffraction is a nondestructive technique that can be used to determine the positions of atoms in crystalline materials. Other uses are phase identification and quantitation, residual stress measurements, and average particle-size estimations for Crystalline materials. Since neutrons possess a magnetic moment, neutron diffraction is sensitive to the ordering of magnetically active atoms. It differs from many site-specific analyses, such as nuclear magnetic resonance, vibrational, and X-ray absorption spectroscopies, in that neutron diffraction provides detailed structural information a v e r q d over thousands of A. It will be seen that the major differences between neutron diffraction and other diffraction techniques, namely the extraordinarily 648


Chapter 11

I Figure 1

Bragg diffraction. A reflected neutron wavefront ( D , , b ) making an angle 0 with planes of atoms will show constructive interference (a Bragg peak maxequals an ima) when the difference in path length between D, and 4(-1 = dsin 8. integral number of wavelengths A. From the construction,

greater penetrating nature of the neutron and its direct interaction with nuclei, naturally lead to its superior usage in experiments on materials requiring a penetration depth greater than about 50 pm. Neutron diffraction is especially well suited for structural analysis of materials containing atoms of widely varying atomic number, such as heavy metal oxides. Basic Principles

Like X-ray and electron diffraction, neutron diffraction is a technique used primarily to characterize crystalline materials (defined here as materials possessing longrange order). The basic equation describing a diffraction experiment is the Bragg equation:

h = 2dsin0

(1)

where drepresents the spacing between planes of atoms in the material in the neutron beam, h is the wavelength of the impinging neutron wavefront, and 28 is the diffracting angle. The difiaction geometry is illustrated in Figure 1. Inspection of the figure demonstrates that a diffraction maxima (a B r a g peak) is observed when there is constructive interference of the reflected neutron wavefront. The intensities of the Bragg peaks depend strongly upon the nature and number of atoms found lying in the planes responsible for the maxima. Consequently, a diffraction pattern can be obtained by fixing the wavelength of the neutron wavefront and scanning 11.1

Neutron Diffraction

649

X-rays

Neutrons

0

0

H

C

0

Ti

Ni

YO00 000

49

0

5 0 0

Figure 2

Fe

g

0

62

Scattering physics of X rays and neutrons

the angle 0, or alternatively, by ffiing 0 and scanning a range of neutron wavelengths. It will be seen that both of these modes of operation are used at modern neutron sources. It should be obvious from Figure 1 that if one wishes to probe Cspacings on the order of atomic spacings (A) that wavelengths of the same length scale are required. Fortunately, X rays, electrons and “thermal”neutrons share the feature of possessing wavelengths of the appropriate size. An important difference between neutron and X-ray diffraction is the way in which neutrons and X rays interact with matter. X rays are scattered primarily by electrons. Consequently, an X-ray diffractionpattern reflects the distribution ofthe electron density within a solid. Conversely, neutrons are scattered by nuclei, and the resultant diffraction pattern reflects nuclear distributions. Since the physics of the scattering differs significantly so does the sensitivity of each technique to various elements. While X-ray scattering from an element is roughly proportional to the local electron density (and the atomic number of the target atoms), neutron scatteringis nucleus-dependentand can vary erraticallyas one proceeds through the periodic table. For this reason, X-ray diffraction analysis of heavy metal oxides (such as Bi,O,) provides information primarily about the metal atoms, whereas neutron diffraction analysis yields detailed positional information for all elements approximately equally. One further important difference between neutron and X-ray diffraction is the former‘s sensitivity to magnetic structure. The magnetic moments of neutrons 650


Chapter 11

interact with the magnetic moments of target atoms, whereas this interaction is much weaker (- 104) for X rays. The interaction strength is proportional to the magnetic moments of the atoms in the material, and depends on their orientation relative to the neutron moment. These features make neutron diffraction the best technique for probing the spatial arrangement of magnetic moments in magnetic materials. Experimental Considerations

Another major difference between the use of X rays and neutrons used as solid state probes is the difference in their penetration depths. This is illustrated by the thickness of materials required to reduce the intensity of a beam by 50%. For an aluminum absorber and wavelengths of about 1.5 A (a common laboratory X-ray wavelength), the figures are 0.02 mm for X rays and 55 mm for neutrons. An obvious consequence of the difference in absorbance is the depth of analysis of bulk materials. X-ray diffraction analysis of materials thicker than 20-50 pm will yield results that are severely surface weighted unless special conditions are employed, whereas internal characteristics of physically large pieces are routinely probed with neutrons. The greater penetration of neutrons also allows one to use thick ancillary devices, such as furnaces or pressure cells, without seriously affecting the quality of diffraction data. Thick-walled devices will absorb most of the X-ray flux, while neutron fluxes hardly will be affected. For this reason, neutron diffraction is better suited than X-ray diffraction for in-situ studies. A less obvious consequence of the difference in absorbance between X rays and neutrons is the large difference in the sizes of facilities using the two types of radiation (primarily for reasons of safety). While only a few millimeters of metal are required to assure the safety of workers near an X-ray source, several meters of absorbing material (usually steel, concrete, or boron-containing materials) are required around neutron sources. Because of the shielding requirement, neutron sources and instruments are orders of magnitude larger than the corresponding X-ray devices. While this leads to much greater expense for neutron sources, it also allows the analysis of larger samples. For example, railroad rails and large-circumference pipes have been analyzed for residual stress at the nuclear reactor at Chalk River, Ontario. This work could not have been done on a standard X-ray diffracto-

Neutron Sources

Two types of sources are used. Originally developed in the 1940s, nuclear reactors provided the first neutrons for research. While reactors provide a continuous source of neutrons, recent developments in accelerator technology have made possible the construction of pulsed neutron sources, providing steady, intermittent neutron beams. 11.1

Neutron Diffraction

651

Name

Location

Type

HFBR

Brookhaven National Laboratory, USA

Reactor

HFIR

Oak Ridge National Laboratory, USA

Reactor

HFR

Institute Laue Langevin, France

Reactor

IPNS

Argonne National Laboratory, USA

Spallation

ISIS

Rutherford-Appleton Laboratory, UK

Spallation

LANSE

Los Alamos National Laboratory, USA

Spallation

NBS

National Institute of Standards

Reactor

and Technology, USA MURR

University of Missouri, USA

Reactor

OWR

Los Alamos National Laboratory, USA

Reactor

ORR

Oak Ridge National Laboratory, USA

Reactor

Some neutron sources.

Table 1

Within nuclear reactors, neutrons are a primary product of nuclear fission. By controlling the rate of the nuclear reactions, one controls the flux of neutrons and provides a steady supply of neutrons. For a diffraction analysis, a narrow band if neutron wavelengths is selected (furing h) and the angle 20 is varied to scan the range of dvalues. Pulsed sources use a process called palkztiun. If a high-energy pulsed beam of protons impinges upon a heavy metal target, a rather complex series of nuclear excitations and relaxations results in a burst of high-energy neutrons from the target. Since the spallation process occurs rapidly (in less than 1 p),a pulsed source can be operated at 30-120 H z (30-120 pulses per second), providing a steady, intermittent source of neutrons. Then, rather than select a narrow wavelength range for diffraction analysis, the full spectrum of neutron wavelengths are used in the h diffraction experiment. Neutron wavelengths vary predictably ~ t momentum according to the equation

A = - ht

(2)

i?d

where h is the neutron wavelength, his Planck's constant, tis time, m is the neutron mass, and lis the neutron flight path. By noting the time of flight of a detached neutron, its wavelength can be deduced. The difference between the neutron difiaction experiment performed at a steady-state (reactor) or a pulsed source are illustrated in Figure 3. Despite the 652


Chapter 11

NEUTRON STEADY-STATE-METHOD

NEUTRON TIME-OF-FLIGHT METHOD Detector

Contlnuaus Source

At Sample At Source

I(S)

SI

Figure 3

Comparison of nuclear reactor and pulsed spallation sources. For reactor sources (steady-state method), a narrow band of wavelengths is selected with a monochromator crystal and the scattering angle (28,) is varied t o scan dspacings. Pulsedsources (time-of-flight method) use almost the entire available neutron spectrum, fix the scattering angle (28,). and simuttaneously detect a neutron while determining its time of flight.

major differences in the design and instrumentation, the quality of data and usable neutron intensities for the two sources are comparable. Some currently available neutron sources are listed in Table 1. Use of Neutron Diffraction Neutron diffraction is particularly well suited for use in 1 Structural investigations of heaty metal oxides (particularlywhen oxygen positions or occupancies are important) 2 In-situ analysis 3 Determinations of magnetic ordering in crystalline materials 4 Bulk analysis of physically large pieces. Specific examples of the first and third uses are given below. 11.1

Neutron Diffraction

653

0

Slag

o Titanium Oxide 0

Iron

LI L 600

Time (minutes) at 1000°C Figure 4

Plot of time-resolved decomposition of t-hnium-enrichedslags as extracted from neutron diffraction data collected at 1000"C.

InSitu Analysis

Figure 4 illustrates the results of neutron diffraction data collected at 1000° C as a function of time. The data were collected during the thermal degradation of commercial titanium slags.' The slags are produced by a smelting process used to enrich the titanium content of ilmenite ore. The purpose of the diffraction experiment was to determine the growth of particular undesirable phases as a function of time at the decomposition temperature of the slag. In addition to providing information about the type and number of phases that appeared during decomposition, it was also highly desirable to obtain detailed structural information about the newly appearing phases, as well as the changing nature of the decomposing phase. Each set of data points for a particular time in Figure 4 represents a neutron diffraction pattern collected in a 15-minute period. Each of the data sets was analyzed to quantify the phases present, determine the identities and locations of metal atoms within each oxide phase, and accurately measure the unit cell parameters (an indication of the phase composition). Examples of the raw neutron diffraction data from which the usell data given in Figure 4 have been extracted are given in Figure 5. In Figure 5, peaks due to two of the decomposition products, titanium oxide (TiOz) and iron metal, are marked; examination of the figure illustrates the utility of diffraction data. The appearance of Bragg peaks due to newly formed phases are clearly distinguished from those of the parent compound. 654


Chapter 11

> n I-

m

z W I-

3

1.5

1.7

1.9

2:;

2.3

2.5

2.7

2.9

0 SPACING

Figure 5

Raw diffraction data at the start (bottom) and completion (top) of the in-situ decomposition of slag experiments. Most of the peaks in the pattern are due to the parent slag phase. Bragg peaks due to titanium oxide (TIand iron metal (Fe) are marked.

It can be seen from Figure 4 that two distinct reaction pathways describe the decomposition of the slag. From the beginning of decomposition up to approximately 150 min, a pathway that results in the production of titanium oxide dominates the chemistry. After this initialization period, the decomposition rate diminishes greatly, and metallic iron formation becomes important. Additional features of the decomposition (not illustrated in Figures 4 and 5) also resulted from analysis of the neutron diffraction data. The most important of these was that a specific atomic arrangement of the titanium and iron atoms was required before decomposition could occur, and that a certain minimum temperature was required for this rearrangement. Knowledge of this atomic shifiing and the temperature r e q ~ r e dfor its occurrence led to an understanding of the maximum temperature above which slags would begin to decompose. Supeffionducting Oxides

One of the most exciting and perhaps unexpected discoveries in science within the last decade has been the observation of superconductivity (the complete absence of resistivity to electric current) in metal oxides at temperature I90 K. This tempera11.1

Neutron Diffraction

655

ture range is particularly important because it can be reached with readily available liquid nitrogen (77 K). Important structural features of metal oxide superconductors have been revealed largely by the application of neutron diffraction. In a sense, a superconductor is an insulator that has been doped (contains random defects in the metal oxide lattice).2 Some of the defects observed via neutron diffraction experiments include metal site substitutions or vacancies, and oxygen vacancies or interstituals (atomic locations between normal atom positions). Neutron diffraction experiments have been an indispensable tool for probing the presence of vacancies, substitutions, or interstituals because of the approximately equal scatteringpower of all atoms. Studies of the superconducting phase Y B ~ ~ C U exemplify ~ O ~ - ~this point. Insitu neutron diffraction analysis revealed that during the synthesis of this material (above 900" C) oxygen vacancies occur and that the composition at this temperature is close to YBa2Cu306 ( x = 1). Upon cooling in an oxygen atmosphere, the oxygen vacancies are filled to form the superconducting material YBa2Cu307+, raising the average oxidation state of the copper above +2. These results are presented in Figure 6. The figure shows that as the temperature is reduced from about 600" C to room temperature, the occupancy of one oxygen site-01, located at (O,vZ,O)in the unit cell-approaches loo%, while that of a second oxygen atom05, located at (Yz,O,O)-tends toward 0. It has been shown that the superconductivity of this metal oxide is a hnction of its oxygen content, and therefore a hnction of the partial occupancies of 0 1 and 05. An advantage enjoyed by neutron diffraction over X-ray diffractionwas outlined in the introduction. Since X rays are scattered by electrons, X-ray diffraction data from the YBa2C~307-~system are mainly sensitiveto the metal atom positions and occupancies, and much less sensitiveto the oxygen atoms. Hence the key features of the Y B ~ ~ C U superconductor-oxygen ~O~-~ vacancies-would not be apparent from the analysis of X-ray diffraction data. However, since neutrons are scattered approximately equally by all atoms, a neutron diffraction experiment is very sensitive to the structural features of importance in this system. An additional advantage of neutrons over X rays in work of this type is the need for in-situ data. To understand the role of oxygen vacancies in the Y B ~ ~ C Usystem, ~ O ~it_was ~ necessary to collect diffraction data over a wide range of temperatures and ox)%en partial pressures. The greater penetrating nature of the neutron is well suited to the use of special equipment required for these types of experiments, since the neutrons can pass right through such equipment. Another example of the use of neutron diffraction to understand the role of atomic vacancies in producing a superconducting metal oxide phase is work that has been perfbrmed on B ~ o ~ K o ~ B This ~O work ~ . demonstrates that at the synthesis temperature (700" C), under the proper conditions, oxygen vacancies are created to allow the formation of the parent phase with bismuth largely in the +3 oxidation state. The presence of the vacancies allows the incorporation of potassium in the 656


Chapter 11

1.0

[a) 100% oxygen

0.9

*

$

' 8

0.8 0.7 0.6

0.5 Q) *

3

0.4

-20

0.3

0

0.2

IA

0.1

-

c,

!!

0.0 -0.1

~

I

-

200

I

400

'

I

600

'

800

I

'

1 DO

Temperature ("C) Figure 6

Site occupancies for two ofthe oxygen atoms (01and 05)in the YBa2Cu307-x superconductor as a function of temperature. The site occupancies resulted from an analysis of in-situ neutron diffraction data. Reprinted by permission from Jorgenrsen and Hinks?

structure. As the temperature is reduced, the oxygen vacancies are filled, and because the potassium atoms lose their mobility at lower temperatures, the overall structure remains intact, producing a phase with bismuth in an unusually high oxidation state. Magnetic Thin Films

Neutron diffraction is a powerful probe of the magnetic structure and ordering in magnetic thin films. Rare earth thin films and multilayers (materials having a repeating modulation in chemical composition) present an interesting class of materials, and neutron diffraction has been instrumental in elucidating their magnetic ~tructure.~ For multilayers of Dy, Er, and Gd alternating with Y, neutron diffraction has shown that the magnetic order is propagated through the intervening nonmagnetic Y layers. For DY-on-Y multilayers, it was found that the magnetidly ordered state was an incommensurate helical antiferromagnetic state. That is, the magnetic moments in each basal plane are ferromagnetically aligned, but somewhat 11.1

Neutron Diffraction

657

rotated between adjacent basal planes. Although this is similar to bulk Dy, the temperature dependence of the rotation, or turn, angle is different than in bulk Dy. It would be difficult or impossible to determine this microscopic information using a technique other than neutron diffraction.While the thickness of the magnetic films in these measurements was +4000A, rare earth films as thin as -500 A and transition metal oxide films as thin as -5000 A can be analyzed. For multilayers, neutron measurements at very low angles are also usell in characterizing magnetic order; these are described in the article on neutron reflectivity. Conclusions

Historically, due to the general unavailability of neutron sources, neutron diffraction has been a rather esoteric technique. Fortunately, the neutron users’ community has expanded over the last decade, and concerted programs encouraging new users at many facilities have extended the use of the technique into the general scientific community. While neutron diffraction may never become a routine analytical tool, data collection times for studies requiring its use usually can be found It has been shown that neutron diffraction offers the same kind of information that other diffraction techniques offer, namely atomically resolved structure determination and refinement, as well as phase identification and quantitation. Other uses not described herein include residual stress measurements, and determinations of average particle sizes for crystalline materials. The major advantage of neutrons with respect to the more readily available X rays lies in the greater penetrating power of the neutron, and the approximately equal scattering ability of nuclei. These features make neutron diffraction the proper choice when in-situ measurements, bulk penetration, or site occupancies of atoms are required. Related Articles in the Encyclopedia

XRD and Neutron Reflectivity References 1

2

3

4

Details of the thermal decompositionof commercialslags can be found in R G. Teller, M. R Antonio, A. Grau, M. Guegin, and E. Kostiner. J Solid State Cbem. 1990. Discussion of neutron diffraction studies of superconductorswas largely taken from J. D. Jorgensen and D. G. Hinks. Neutron N m s . 24,1,1990. For further discussion of neutron sources, see R B. Von Dreete. Reviews in Minerabgy. Volume 20: Modern P o d r Difiaction. 333,20,1990. For a detailed discussion of pulsed neutron sources, see J. D. Jorgensen and J. Faber. I M S - I . Proceedings of the Sixth International Cohboration

658


Chapter 11

on Advances in Neutron Sources. Argonne National Laboratory technical 5

6

7

report ANL-82-80, 1983. Important concepts in neutron diffraction can be found in G. E. Bacon. Neutron Dzfiaction. Clarendon Press, third edition, 1975. The general principlesof diffractioncan be found in numerous books, for example, B. D. Cullity. E h e n t s ofX--Ray Dzfiaction. Addison-Wesley, New York, second edition, 1978. For a review, see J. J. Rhyne, R W. Erwin, J. Borchers, M. B. Salmon, R Du, and C. l? Flynn. Physica B. 159, 1 1 1, 1989.

11.1

Neutron Diffraction

659

11.2

Neutron Reflectivity THOMAS P. RUSSELL

Contents

Introduction Basic Principles Instrumentation Specimen Considerations Examples Conclusions

Introduction Neutron reflectivity offers a means of determining the variation in concentration of a material’s components as a function of depth from the surface or at an interhce buried within the material, with a resolution of -1 nm. Because of the large neutron contrast between hydrogen and deuterium, one may highlight a particular component through isotopic labeling with deuterium without substantialiy altering the thermodynamics of the system. Thii,however, normally means that reflectivity studies are relegated to the investigation of model systems that are designed to mimic the behavior of rhe system of interest. Other technique-for example, dynamic secondary ion mass spectrometry or forward recoil spectrometry-that rely on mass differences can use the same type of substitution to provide contrast. However, for hydrocarbon materials these methods attain a depth resolution of approximately 13 n m and 80 nm, respectively. For many problems in complex fluids and in polymers this resolution is too poor to extract critical information. Consequently, neutron reflectivity substantially extends the depth resolution capabilities of these methods and has led, in recent years, to key information not accessible by the other techniques. 660


Chapter 11

An additional advantage to neutron reflectivity is that high-vacuum conditions are not required. Thus, while studies on solid films can easily be pursued by several techniques, studies involving solvents or other volatile fluids are amenable only to reflectivity techniques. Neutrons penetrate deeply into a medium without substantial losses due to absorption. For example, a hydrocarbon film with a density of lg’cm3 having a thickness of 2 mm attenuates the neutron beam by only 50%. Consequently, films several in thickness can be studied by neutron reflectivity. Thus, one has the ability to probe concentration gradients at interfaces that are buried deep within a specimen while maintaining the high spatial resolution. Materials like quartz, sapphire, or aluminum are transparent to neutrons. Thus, concentration profiles at solid interfaces can be studied with neutrons, which simply is not possible with other techniques. The single most severe drawback to reflectivity techniques in general is that the concentration profile in a specimen is not measured directly. Reflectivity is the optical transform of the concentration profile in the specimen. Since the reflectivity measured is an intensity of reflected neutrons, phase information is lost and one encounters the age-old inverse problem. However, the use of reflectivity with other techniques that place constraints on the concentration profiles circumvents this problem. The high depth resolution, nondestructive nature of thermal neutrons, and availability of deuterium substituted materials has brought about a proliferation in the use of neutron reflectivity in material, polymer, and biological sciences. In response to this high demand, reflectivity equipment is now available at all major neutron hcilities throughout the country, be they reactor or spallation sources. Basic Principles

Considering Figure 1, radiation incident on a surfice (light, X rays, or neutrons) will be reflected and refracted at the interface between the two media provided there is a difference in the index of refraction. In the case of neutrons and X rays, the refractive index of a specimen is slightly less than unity and, to within a good approximation, is given by n = l-S+iP

(1)

The imaginary component of the refractive index is associated with absorption. In general, the absorption for thin films is not significant and, consequently, p can be ignored. However, for materials containing the elements Li, ByCd, Sm, or Gd, where the absorption coefficient is large, must be taken into account and the refractive index is imaginary. The real component of the neutron refractive index 6 is related to the wavelength h of the incident neutrons, the neutron scatteringlength (a measure of the extent to which neutrons interact with different nuclei), the mass density and the atomic 11.2


66 1

\ Incident Radiation

\

/

Reflected Radiation

Vacuum ( 0 )

T

d

Specimen (1)

Substrate (2)

Figure 1

Schematic diagram of the neutron reflectivity measurement with the neutrons incident on the surface and reflected at an angle 8, with respect t o the surface. The angle €I2 is the angle of refraction. The specimen in this case is a uniform film with thickness d, on a substrate.

number of the components comprising the specimen. Values of the neutron scattering length for all the elements and their isotopes are tabulated.' The neutron scattering length does not vary systematically with the atomic number. This is shown in Figure 2. As can be seen, isotopes of a given element can have markedly different neutron scattering lengths while two different elements with vastly different atomic numbers can have similar scattering lengths. In fact, the difference between the proton and the deuteron provides one of the largest differences and offers the most convenient manner of labeling materials for neutron reflectivity studies. In Table 1 values of 6 are given for some common inorganic and organic compounds. First notice that 6 is on the order of lo4. Therefore, the neutron refractive index differs from unity by only a small amount. With the exception of HZO and

Figure 2

662

Variations in the neutron scattering amplitude or scattering length as a function of the atomic weight. The irregularities arise from the superposition of resonance scattering on a slowly increasing potential scattering. For comparison the scattering amplitudes for X rays under two different conditions are shown. Unlike neutrons, the X-ray case exhibits a monotonic increase as a function of atomic weight.


Chapter 11

H20

1 .oo

-0.168

D2O

1.11

C6H6

-0.212

-

-

1.914

2.41

2.19

0.0089

0.879

1.74

0.445

0.943

0.0038

0.946

7.99

2.043

2.021

0.0082

(CHz),

0.95

-0.08

-0.128

-

-

(CD2),

1.08

1.99

3.067

2.477

0.0101

(CgH,),

1 .o

2.32

0.509

1.01

0.0041

(CgDg),

1.08

10.656

2.336

2.16

0.0088

Silicon oxide

Si02

2.32

1.58

1.371

1.655

0.0067

Silicon

Si

2.32

0.42

0.791

1.26

0.0051

Water

Benzene

Polyethylene

Polystyrene

tcalculated for neutrons with k = 1.5 k In this l i t , p is the mass density, Z 6; is the sum of scattering lengchs of the atoms comprising the molecule, 6 is the reai part of the refracrive index, Bc is the critical angle, and is the critical neutron momentum.

Table 1

Important neutron reflectionparametersfor some common materials.

polyethylene (CH2)* 6 > 0 and therefore n c 1. The refractive index for air or vacuum is unity. From Snell's law it is easy to show that total external reflection occurs when the incidence angle, 8, is less than (26)". Above this critical angle 8c the reflectivity decreases and the manner of this decrease contains all the information pertinent to gradients in the concentration normal to the surface of the specimen. Consider the simple case shown in Figure 1, where we have a uniform film on a substrate with a thickness of 50 nm. For demonstration purposes the film is a deuterated polystyrene on a silicon substrate. The reflectivity profile obtained from such a specimen is shown in Figure 3. The reflectivity is plotted as a function of the where the subscript a0 neutron momentum RAYwhich is proportional to (sin indicates the direction (normal to the surface) in vacuum (0), h is the wavelength and 8 is the incidence angle. Using k,O reduces data to a scale that is independent of 3c and 8 used in an experiment. Below 8c total external reflection is seen, then the reflectivity is seen to decay with a series of oscillations. The oscillations, characterizing the film thickness, arise from interferences between neutrons reflected at the air and substrate interfaces. In general the higher the frequency of the oscillations, the thicker the specimen. 1 1.2


663

I

I

0.02

I

0.04

I

0.06

kz(A-')

Figure 3

A calculated reflectivity profile for a perdeuterated polystyrene film with a thickness of 50 nm on a silicon substrate. The calculation was for a specimen where the interfaces between the specimen and air and the specimen and the substrate were sharp. This causes the reflectivity on average (shown by the The separation distance dashed line) to decrease in proportion to kS4 or between the minima of the oscillations directly yields the thickness of the specimen, as shown.

e.

On average, the reflectivity decays in proportion to O4 or keo4 since both interfaces are sharp. However, if either surfaces is rough, then marked deviations are seen and the reflectivity is damped by a factor of exp{-2kd2d}, where 0 is the rootmean-square roughness. Thus, the reflectivity is very sensitive to surface roughness and to concentration gradients at interfaces. The reflectivity for this simple case can be extended readily to more complex situations where there are concentration gradients in single films or multilayers comprised of different components. Basically the reflectivity can be calculated from a simple recursion relationship that effectively reduces any gradients in composition to a histogram representing small changes in the concentration as a hnction of depth. Details on this can be found in the literature cited.24 For specimens where gradients in the magnetic moment are of interest, similar arguments apply. Here, however, two separate reflectivity experiments are performed in which the incident neutrons are polarized parallel and perpendicular to the surface of the specimen. Combining reflectivity measurements under these two polarization conditions in a manner similar to that for the unpolarized case permits the determination of the variation in the magnetic moments of components parallel and perpendicular to the film surface. This is discussed in detail by Felcher et and the interested reader is referred to the literature.

664


Chapter 11

Instrumentation

The neutron’s momentum can be varied by changing h or 8. With a fured wavelength the angle of incidence can be changed by rotating the sample. This, typically, is the situation one encounters at steady state nuclear reactors, where a specificwavelength of the neutrons emanating from the reactor core is selected with a monochromator. Such facilities are available at the Oak Ridge National Laboratory, the National Institute of Standards and Technology, and the Brookhaven National Laboratory in the United States, and at the Institut Laue Langwin (Grenoble) and the Kernforschungsanlagefacility (Julich)in Europe. Here, a collimated beam of monochromatic neutrons impinges on the surface of the specimen at an angle 0 and a detector is placed behind a set of slits at an angle 28 with respect to the incident beam (this is termed the specular condition). The geometry is shown in Figure 1. Both the specimen and detector are rotated synchronously at 8 and 28, respectively, to measure the reflected neutrons as a function of 8. Since 3c is known, 8 is equivalent to &o. Normalization of the reflected intensity against the incident intensity yields the reflectivitydirectly. Spallation sources are an alternative means of generating neutrons. As opposed to a reactor source, a target is bombarded with pulses of high-energy charged particles, for example, protons on a uranium target. Neutrons with a broad energy or wavelength distribution are generated at the target, passed through a cold moderator, and delivered into the experimental area. The velocity of a neutron is proportional to its energy and inversely proportional to its wavelength. Thus, knowing the time at which the pulse of protons hits the target and the distance to the source, we have that the time for a neutron to reach the detector is proportional to its wavelength. Hence the term time ofjight is used. Argonne National Laboratory and the Los Alamos National Laboratory in the United States, and Rutherford Appleton Laboratories in Europe have reflectometersthat are based on this principle. At spallation sources, ka is varied by wavelength because pulsed streams of neutrons with a range of wavelengths are delivered onto the specimen surface at an angle 8. Knowing the incident distribution of wavelengths and measuring the distribution of wavelengths reflected at an angle 8 with respect to the surface furnishes directly the reflectivity of the specimen. The beauty of the time-of-flight measurements is that there are no moving parts to the reflectometer. Unlike the fixed-wavelength spectrometers, with time-of-flight spectrometers exactly the same area of the specimen is measured for all values of 40.This is very important if the sample is not uniform across its surface. One of the key advantages to time-of-flight reflectometers comes in the measurement of fluid surfaces. Simply delivering the neutrons onto the fluid surface at a fured angle (without moving the specimen) and detecting the reflected neutrons yields the reflectivityprofile. 11.2


665

Specimen Considerations

The measurements of concentration gradients at surfaces or in multilayer specimens by neutron reflectivity requires contrast in the reflectivity for the neutrons. Under most circumstances this means that one of the components must be labeled. Normally this is done is by isotopic substitution of protons with deuterons. This means that reflectivity studies are usually performed on model systems that are designed to behave identically to systems of more practical interest. In a few cases, however (for organic compounds containing fluorine, for example) sufficient contrast is present without labeling. Neutron reflectivity measures the variation in concentration normal to the surface of the specimen. This concentration at any depth is averaged over the coherence length of the neutrons (on the order of 1 pm) parallel to the surface. Consequently, no information can be.obtained on concentration variations parallel to the sample surfice when measuring reflectivity under specular conditions. More importantly, however, this mandates that the specimens be as smooth as possible to avoid smearing the concentration profiles. Typically specimens for reflectivity measurements are prepared on flat, smooth, rigid substrates. For example, these substrates can be polished fused silica, quartz, or silicon. It is important, however, that the substrates be thick to avoid distortions of the specimen when mounted in the reflectometer. Any curvature or bowing will increase the divergence of the incident beam and result in a deterioration of the resolution. The substrates usually range from 5 cm to 10 an in diameter. Such large specimens are required because experiments are performed at small angles of incidence and it is necessary to intercept the entire beam with the specimen. For example, consider the projection of a beam 0.1 mm in size onto a specimen at an angle of 2 mrad (- 0.1 1O). Under such conditions, the incident beam will illuminate 5 cm of the surface. For specimens that are dominantly protonated, the critical angle is smaller and the projection is even larger. The large diameter of the specimensplaces stringent requirements on the sample’s preparation, but it is not difficult to achieve uniformity for such large specimens and the preparation is routine.

The first studies on the use of neutron reflectivity appeared approximately 20 years ago and dealt with the investigation of Langmuir-Blodgett films. Surprisingly, the utility of the technique for the investigation ofother materialswas not hlly realized and its use and availability virtually ceased. Only in the last four years has there been a resurgence of the use of neutron reflectivity, and now there are reflectometers available throughout the United States and abroad. This resurgence stems from the focus of the scientific community on surface and interfacial phenomena. In 666


Chapter 11

hct, interfacial behavior of polymeric materials has provided a rich area where the true power of neutron reflectivity has been brought to bear. There is no question, though, that surface and interfacial problems in other organic, inorganic and magnetic materials can be studied by neutron reflectivity. These areas have been discussed at length in the literature.4-6 For the purposes of illustration, we shall focus on two specific problems in polymers that demonstrate the capabilities of reflectivity. In numerous applications of polymeric materials multilayers of films are used. This practice is found in microelectronic, aeronautical, and biomedical applications to name a few. Developing good adhesion between these layers requires interdiffusion of the molecules at the interfaces between the layers over size scales comparable to the molecular diameter (tens of nm). In addition, these interfaces are buried within the specimen. Aside from this practical aspect, interdiffusion over short distances holds the key for critically evaluating current theories of polymer diffusion. Theories of polymer interdifhion predict specific shapes for the concentration profile of segments across the intedace as a function of time. Interdiffusion studies on bilayered specimen comprised of a layer of polystyrene (PS) on a layer of perdeuterated (PS) d-PS, can be used as a model system that will capture the fundamental physics of the problem. Initially, the bilayer will have a sharp interface, which upon annealing will broaden with time. Neutron reflectivity is ideally suited to this problem, since concentration profiles can be resolved on the nanometer level and since, for an infinitely sharp interhce, Rkz; will approach asymptotically a constant value. In addition, neutron reflectivity is nondestructive and multiple experiments can be performed on the same specimen. Figure 4 shows a plot of % ; as a function of 40 for a bilayer of protonated polystyrene (h-PS) on a layer of, d-PS prepared as described. Near the critical angle, Rko4 reaches a maximum value then decreases, approaching an average constant value of 3 x 1O-'OA4. Thus, the initial interface is sharp. Evident in the reflectivity profile are a series of oscillations with frequencies characterizing the thicknesses of the h-PS and d-PS layers and the combined thickness. The solid line in the figure corresponds to a reflectivity profile calculated assuming a 67.5-nm layer of h-PS on an 185-nm layer of d-PS with an interfacial width of only 1 nm. Upon heating the bilayer for two minutes at 105.5'C, where the glass transition ; temperature of PS is 100' C,interdiffusion occurs. As shown by the plot of & versus 40 (offset by a factor of IO), the overall features of the reflectivity remained unchanged with the exception that the asymptotic limit decreased to 1.4 x lO-'OA4. The solid line in the figure was calculated using the layer thicknesses mentioned above, but the interface was characterized now by an error h c t i o n with a width of 3 nm. Combining these measurements with others at longer times and at different temperatures proved that reptation (the movement of the polymer along its own contour), coupled with rapid motions of the polymer chain between entanglements, quantitatively describes the interdiffusionprocess. 11.2


667

Figure 4

The reflectivity multiplied by kzo4as a function of 140 for a bilayer of normal polystyrene on perdeuteratedpolystyrenebefore and after heating at 105.5' C for 2 minutes. The data is plotted in this manner since for sharp interfaces ~ k , ~is' a constant at large k-

The second example deals with symmetricdiblock copolymers. Diblock copolymers are finding widespread use as compatibilizingagents,adhesion promoters and surfice-modifying agents. Diblock copolymers are comprised of two chemically different polymer chains covalently bonded together at one point. In cases where the length of the blocks are equal, lamellar domains of each component comparable to the size of the polymers are formed at the junction points between the two blocks at the interfices. The chemical differencesbetween the blocks causes a preferential segregation ofone of the components to the air or substrate interfaces, in thin films, which leads to an orientation of the lamellar domains parallel to the frlm surfice. Figure 5 shows the neutron reflectivity profrle of a diblock copolymer of PS and polymethylmethacrylate (PMMA), where the PS block is labeled with deuterium. The reflectivity profile exhibits five clearly resolved maxima characterizing the period of the lamellar microdomains. A high-frequency oscillation, characteristic of the total specimen thickness, is also seen. Note that below the critical angle (So 0.006 A-1)the reflectivity is not unity, due to the finire size of the specimen. The solid line in the figure was calculated using the scatteringlength density or concentration profile shown in the inset. This model is comprised of PS layers having a 668


Chapter 11

Figure5

Neutron reflectivity of a symmetric diblock copolymer of PS and PMMA annealed at 160" C for 24 hours under vacuum. The inset contains scattering length density profiles used t o calculate the corresponding reflectivity profiles.The solid line represents the reflectivity profile calculated using the scattering length density (or concentration) profile as a function of distance zfrom the air surface where a hyperbolic tangent describes the interface between the layers. The dashed line was calculatedusing a linear gradient between the microdomains. This small change, as can be seen, produces a significant change in the reflectivity profile.

thickness of 21 nm and PMMA layers having a thickness of 18.8 A. The interface between the layers is characterized by a hyperbolic tangent function with an effective width of 5.0 k 0.3 nm. At the air-copolymer and copolymer-substrate interfaces, the adjacent PS and PMAM layers, respectively, are one-half the thickness of those seen in the bulk. The major importance of these results comes in the precision to which the interface between the PS and PMMA layers can be determined. This far exceeds that possible by other techniques. To emphasize this, a second reflectivity profile (shown as the dashed line) was calculated assuming a linear gradient at the interface. While the differences in the segment density profiles are small, the differences in the reflectivity profile are pronounced, clearly demonstrating the details resolvable by neutron reflectivity. Conclusions

Neutron reflectivity provides a depth resolution of -1 nm and fills an important gap in the resolution between X-ray photoelectron spectroscopy and ion-beam techniques. In this regard, neutron reflectivity promises to play a decisive role in the investigation of solid materials. Equally important is the fact that reflectivity meas11.2


669

urements can be used to study liquid specimens, which is not possible with other techniques requiring high vacuum. Many problems on the swelling and dissolution of materials with low molecular weight solvents, the adsorption of materials onto a surface or at the liquid-air interface or liquid-liquid interface, or transport through membranes in solution on specific interactions of materials with interfaces in the presence of a solvent can be addressed by neutron reflectivity. Theoretical and experimental advances are being made on understanding reflectivity under nonspecular conditions. This permits the determination of correlations in composition parallel to the specimen’s surface thereby adding a new dimension to existing capabilities. Studies under nonspecular conditions promise to be quite important in elucidating systems containing inhomogeneities parallel to the sample’s surfice. From this will stem a quantitative characterization of surface roughness, curvature of interfaces, capillary waves, and the configurations and conformations of molecules at surfaces and interhces. Related Articles in the Encyclopedia

Neutron Diffraction References I

2

3 4

5

6

G. E. Bacon. Neutron Dzfiaction. (3rd edition) Clarendon Press,Oxford, 1975. 0. S. Heavens. OpticalPropertiesof Thin Solid Films.Buttemorths, London, 1955. J. Lekner. Theory of Rej9ection. Nighoff,Dordrecht, 1987.

T. E Russell. Mut. Sei. Rep. 5, 171, 1990. G. I? Felcher, R 0.Hilleke, R. K. Crawford, J. Haumann, R. Kleb, and G. Ostrowski. Rev. Scz. Instrum. 58, GOY, 1987. J. Penfold, R .C. Ward, and W. G. Williams. /. Pbys. E20, 1411, 1987.

670


Chapter 11

NAA

11.3

Neutron Activation Analysis T I M Z. H O S S A I N

Contents

Introduction Basic Principles Sample Requirements Quantification Applications Limitations Conclusions

Introduction

Neutron activation analysis (NAA) is a sensitive analytical technique for measuring trace impurities in a wide variety of materials. NAA has been used for trace analysis in polymers, biological materials, and geological and environmentalsamples. It has been used successfullyto study the role of trace elements in Alzheimer's Disease by measuring impurities in human brain tissue. Trace impurities in air particulates have been measured to correlate emissions from fossil fuel power plants. Since there are many excellent texts describing the applications of NAA in these fields, the present article will be devoted entirely to NAA of semiconductor materials. NAA is highly suited for elements of interest to semiconductor manufacturing. For example, NAA can detect Cu, Zn, and Au in Si with detection limits of 7 x 1x 10l2,and 3 x lo7 atoms/cc, respective1y.l Results obtained using this method usually provide bulk concentrations, however, surface-sensitive measurements can be made by combining chemical etching with NAA. The method is not suitable for depth profiling, such as can be done with SIMS. Neither is there a capability for 11.3

NAA

67 1

I I

Figure 1

1

Schematic of Neutron Activation Analysis.

microspot analysis. The strengh of NAA lies in its ability to perform quantitative bulk analysis at sub-ppb levels. Almost two-thirds of the elements in the periodic table can be analyzed easily. Light elements like H, N, 0,or F are not suitable candidates for NAA, and Al and C1 can be analyzed only with high detection limits in a Si matrix. Most of the transition elements that are of primary interest in the semiconductor industry such as FeyCr, Mn, Coyand Ni, can be analyzed with very low detection limits.' Second to its sensitivity, the most important advantage of NAA is the minimal sample preparation that is required, eliminating the likelihood of contamination due to handling. Quantitative values can be obtained and a precision of 1-5% relative is regularly achieved. Since the technique measures many elements simultaneously, NAA is used to scan for impurities conveniently. It is applicable to plastic packaging materials, where purities with respect to mobile ions, such as C1 and Naycan be checked. In addition, a-particle precursors, such as U and Thycan be determined in solid plastics with sub-ppb detection limits. Basic Principles Irradiation

All NAA experiments are conducted in two steps: irradiation and counting as indicated in Figure 1. Samples are made radioactive by placing them in a neutron field. Typically a research nuclear reactor provides the necessary neutron flux. Elements present in the sample capture neutrons, and often become radioactive isotopes. This part of the experiment is known as irradiation. A typical irradiation in a reac672


Chapter 11

tor core position (where the neutron flux is high) may last about 1-7 days. The total integrated neutron dose received, in general, determines the detection limits obtained. Typical research reactors are capable of delivering 1-5x neutrons/un 2 s. It is important to note that the neutron capture probability, called the cross section 6,is vastly different for various elements. Excellent sensitivity for Au is due largely to its high cross section (6 = 100 barns; 1barn = 1 x cm2). Other elements, such as Pb, have low cross sections and much poorer detection limits. Elements with multiple stable isotopes may produce several radioisotopes that can be measured to assure the accuracy of the analysis. For exam le, Zn has five stable isotopes? The isotope &Zn will produce the radioisotope “Zn, and G8Znwill produce the radioisotope G9Zn.Both of these radioisotopes can provide an independent measurement of the Zn concentration and therefore can be used to check the consistency and quality of the analysis. O n the other hand, GGZnwill produce G7Zn,which is nonradioactive and therefore cannot be used in NAA. At the end of the irradiation, the samples are withdrawn from the reactor and y-ray spectroscopy is carried out. Most ofeen the laboratory performing the y-ray spectroscopy is located in a different city, in which case the samples are shipped and the reactor serves as a neutron source only. Many reactors also have y-ray spectroscopy capability so that measurements can be made at the reactor site as well. Counting, or yRay Spectroscopy

All radioactive isotopes decay with a characteristic half-life. For example, 59Fe decays with a half-life of 45 days, while G4Cudecays with a half-life of 12.6 hours. As a result of the decay, si ature high-energy photons or y rays are emitted from a iven radioisotope. Thus,’Fe emits two rominent y rays at 1099 and 1292 keV, 54Na emits at 1368 and 2754 keV, and G PZn emits at 1115 keV. Compilations of

y rays used in NAA can be found in y-ray tables. The emission of yrays follows,in the majority of cases,what is known as p decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which promptly relaxes by giving off y rays. This is illustrated in Figure 2. For the purpose of the activation analysis these high-energy y rays are used or counted in a spectroscopy mode to obtain concentration values. Energy-dispersive spectroscopy of the y rays is accomplished using a solid state detector constructed of high-puriry single-crystal germanium. The detector is connected via a preamplifier and an amplifier to a multichannel analyzer (MCA). The sample holder and the detector are contained in a lead-shielded chamber. The radioactive sample serves as the y-ray source and is placed in a convenient geometry in front of the detector. The response time of the detector must be fast enough so that each y ray entering the 11.3

NAA

673

5.51 5.22 4.23 4.123

1.369

Figure 2

Decay scheme of %a. The transition energies are in MeV?

detector volume is processed as a single event. The y rays are sorted by energy and counted in appropriate channels of the MCA.A spectrum is generated of intensity (counts) versus energy of the y rays. While the energy of the y ray is used to identify the element, the number of counts provides a measure of the concentration. NAA can achieve ultralow detection limits for most impurities in a Si or Si02 matrix because Si itself has a low neutron capture cross section and roduces radioisotopes having short half-lives. The only significant radioisotope, "Si, has a halflife of 2 6 hours. Radioactivity from this isotope, although initially intense, will therefore be reduced to a negligible intensity after 2-3 days. The impurities of interest, such as Cr and Feyhave long half-lives. By delaying the start of the count, a high signal-to-noise ratio is obtained. This indicates that for other matrices, such as HgCdTe and GaAs, in which both the cross sections and the half-lives are not as favorable, NAA is rendered less practical. On the other hand, organic matrices, such as polymers and biological samples, can be conveniently analyzed because the light elements CyH, 0,and N have low neutron capture cross sections. Sample Requirements

Size requirements are limited by pa+ng considerations for neutron irradiation. Typically, polyethylene or quartz containers are used to contain the sample in the reactor core. For example, Si wafers are cleaved into smaller pieces and flame sealed 674


Chapter 11

in high-purity quartz vials (e.g., suprasil quartz) for irradiation. The size of the irradiation port in the reactor core will typically limit the size of the polyethylene or quartz container. AN handling is done wearing talc-free gloves and using white plastic tweezers. (Avoid the green-colored Teflon-coated stainless steel variety.) An intact full wafer (e.g., having a diameter of 4") usually cannot be inserted into an irradiation port of high neutron intensity (> 1 x 1013 neutrons/ (cm2s), since most ports have diameters of less than 4" It is desirable to perform NAA befbre any metallization steps (with Ti, W, or Cu, for example) have been carried out. This avoids a high intensity of radiation from the irradiated thin metal layer. Other important considerations are the dopants present in the wafer. Both As and Sb become readily activated (with half-lives 26.7 hours for As, and 64 hours and 60 days for the two radioisotopes of Sb), so that samples implanted with high doses ofAs and Sb (> 1 x lo1*atoms/cm2) are not suitable for NAA However, B and P dopants do not pose problems during NAA. Quantification

NAA is a quantitative method. Quantificationcan be performed by comparison to standards or by computation from basic principles (parametricanalysis).A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence),matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standard and Technology Standard Reference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental hctors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations:

where A is the activity in disintegrations/ second; cps stands for coundsecond in a given photopeak &e., a specific y ray); Ed is the efficiency of the detector for the photopeak; is the photon fraction (i.e., the number of y rays/ 100 decays); n is the number of atoms of the element of interest (unknown); Q is the isotopic abundance of the target; fis the neutron flux density; Q is the neutron cross section; h is the decay constant for the radioisotope; ti is the length of irradiation; and tdis the time elapsed since the end of irradiation. Applications

NAA has been used to determine trace impurities in polysilicon, single-crystal boules, silicon wafers, and processed silicon, as well as plastics used fbr packag1ng.+5 11.3

NAA

675

Ag

c

0.00130

c

0.00110

As

0.02400

0.00810

Au

0.00007

0.00005

Ba

Not identified

Not identified

0.00640

< 0.00490

0.0520

c

0.00600

0.00040

c

0.00020

co

c

Cr cs

c

Fe Ga

0.00730

0.00590

cu c

0.44000

Not identified

c

0.25000

Not identified

Hf

c

0.00021

c

0.00015

In

c

0.01800

e

0.05900

Mo

c

0.01300

Not identified

Not identified

0.62000

Na Rb

c

0.01100

c

0.01000

Sb

c

0.00040

c

0.00250

sc

< 0.00005

c

0.00064

Ta

c

0.00008

c

0.00010

Zn

e 0.00150

c

0.00110

Zr

c

0.1800

c

0.13000

Table 1

Comparison of NAA data for polysiliconand single-crystalCZ silicon?

An example of an analysis done on polysilicon and single-crystalCmchralski silicon (CZ) is shown in Table 1. As can be seen, polysilicon, which was used to grow the crystal, is “dirtier” than the CZ silicon. This is expected, since segregation coeficienrs limit the incorporation of each element into the crystal boule during the crystal growth process. All values shown in the table are from bulk analysis. Table 2 shows NAA data obtained in an experiment where surface analysis was accom676


Chapter 11

Impurity surface (4 p)

Concentration( a t o d c c )

Epitaxial film (> 4 p)

AU

1 x 1013

1 x 10l2

cu

7 x 1014

6 x 1013

Na

3x

7 x 10'3

Sulface Concentrations of impurities on a Si wafer, as measud by N U . *

Table 2

plished by using chemical etching following irradiation. The wafer surfice is usually etched with a HF/HNO3 /glacial acetic acid mixture and a 2-3 pm layer is etched off by controlling the length of the etching time. Both the etched wafer and the etchant solution are then measured. Many etchants developed for wafer fabrication can be used for these measurements.Table 3 shows results obtained from NAA of plastic packaging materials. Low detection limits from U and Th are particularly noteworthy. Although the majority of NAA applications have been in the area of bulk analysis, some specializeduses need to be mentioned. One such unique application is the measurement of phosphorus in thin films (about 5000 A) of phosphosilicate (PSG) or borophosphosilicate (BPSG) glasses used in VLSI device fabrication. In this case, Impurity element

PVI (5 lots) (PPbw)

Au

0.1-0.3

0.005-0.05

cu

65-100

110-1800

Cr

Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films

Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films (Materials Characterization Series)

Solid surfaces, interfaces and thin films

Solid Surfaces, Interfaces And Thin Films

Materials Science of Thin Films

Nanostructured Thin Films and Surfaces